Java & J2EE Made Easy

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Java & J2EE Made Easy
PHANI KOSURI

J2EE Consulting Inc.

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For online information and ordering of this book, please go to www.j2eeconsultant.net. The author offers discounts on this book when ordered in quantity. For more information, please contact: Phani Kosuri J2EE Consulting Inc., 8400 Gradington Dr, Westerville, OH 43081

For more than a decade. Because of this mutual benefit to customers and businesses. Java based client server applications over the years have gone though several phases from traditional two-tier applications.preface
In today’s world. the ability to port the code from one operating system to the other. One of the primary reasons for the success of Java and J2EE is the simplicity that it offers and the flexibility. adoption of sophisticated technologies has become the key factor in exploiting the information assets of a business. Though there are billions of technologies that helped business to some extent to address the challenges. Java and J2EE has relentlessly served and serving the enterprise community in building enterprise e-commerce applications. To make Java and J2EE even strong. where fat clients like GUI’s acted as user interfaces for interacting
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. These applications have now become the faces of modern businesses on internet and act as a bridge for customers to reach businesses and vice-versa. Besides this. it is the introduction of Java and J2EE that helped businesses to stand firmly and with authority. On top of the above. to sustain from this immense competition. Java language includes the most powerful built-in libraries that heavily reduce the development time and also makes the application more reliable. With Internet became an integral part of our life. Though Java programming language like any other programming language allows building standalone applications. Therefore. This is what led to the so called client server applications. networking and many more. several communities have been developed and developing to constantly identify potential challenges and incorporating sophisticated solutions to address challenges. every known business irrespective of size is moving into cyber world from our physical world to capture customers attention. One feature that stands on top of all is the portability factor. its true strength lies on the server side where it is used to build highly sophisticated server applications using networking and multi-threading capabilities. At the same time customers are also searching for ways to make life easy by using internet for most of their day to day activities. One of the key challenges today’s e-commerce applications face is sustaining severe competition from other similar businesses. e-commerce applications or enterprise applications are taking off from the runway into internet space. we all know the role of e-commerce applications and how much impact they had on our lives. Java programming language has inherent support for concurrent processing. Java as a programming language is designed to be very simple and yet very powerful.

The solution is nothing but the J2EE platform.with server application to the latest n-tier applications that use the standard web browser for interacting with the server via internet. J2EE platform is a bundle of several ready made technologies with each being used in different areas within the enterprise application. the quest for building better Java based solutions didn’t stop. With enterprises deciding to use internet to do business to increase the customer base. the blend of Java. Few such open source frameworks that have become very popular these days are Struts. This seriously affected the performance and scalability of the application and also resulted in maintenance nightmares. the standard J2EE technologies served the industry and are still serving. Spring and Hibernate. but serves an alternative to some of the standard J2EE technologies within the platform. Over the years. Java applications started getting overloaded due the increase in the number simultaneous requests. Good luck. Open source frameworks don’t reinvent the wheel but try to build a better wheel. Java community got into the act and came with a powerful solution. With open source frameworks also gathering momentum. Though the J2EE technologies provided end to end solutions for building enterprise applications. Starting with Java programming language. It is very important to understand that these frameworks don’t replace the entire J2EE platform. To solve this problem. This quest is what led to the so called open source frameworks. This book covers all the above three. These frameworks are again built from the core J2EE technologies to further simplify the application development.
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. I hope you’ll surely enjoy the journey and have a good time. The entry of J2EE into the world of enterprise application development radically changed the dimensions of enterprise internet computing and has now become almost a de facto standard for building and hosting enterprise internet applications. some used for building business components etc. it slowly takes you though the journey of J2EE and open source. J2EE and open source frameworks has become a killer combination for enterprise internet application development. some technologies are used to build presentation components. For instance.

Chapter 2: Java Language Fundamentals This chapter helps you to understand and write simple Java programs and highlights some of the important Java keywords. Inheritance etc. It talks about how Java achieves platform independence and the types of applications that can be built using Java. this is a great place to start. This is the most important chapter and understanding it makes the journey through Java more interesting and fun. This chapter explains all the OOPS concepts like Encapsulation. Chapter 4: Object Oriented Programming This chapter will demonstrate Object Oriented Programming (OOP) concepts and their implementation in Java. Chapter 5: Referential Polymorphism This chapter teaches you on how to use parent class and interface references. compile and execute basic Java programs using classes. If you don’t have any idea about what Java is all about. It also explains how to install Java and set up the environment for the remaining chapters. You'll surely enjoy this chapter since OOP is fun and its associated terminology is heard in our day to day life. Understanding the concepts in this chapter is utmost important since most of the real world Java applications are based on Object Oriented Design whose primary weapons are inheritance and interfaces. Chapter 3: Packages This chapter demonstrates the idea of using packages to logically organize Java classes. you will be able to know how and when to use inheritance and interfaces in real world applications.What’s in this Book
Chapter 1: Introduction This first chapter serves as a brief introduction to Java and its role in today’s world. This chapter ensures that you understand how to write. By the end of this chapter. as it forms the basis for the next several chapters. Understanding this chapter is important.
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. The ideas and examples in this chapter are totally based on what we learned in the previous chapter. It also explains the usage of various keywords associated with packages and how to implement access control for classes stored within packages.

This chapter will also give you an idea on parsing XML documents using JAXP API. Chapter 7: Core Libraries and Best Practices This chapter explains the important built-in Java class libraries. Understanding this chapter gives you an edge for a successful career in Java. This chapter also explains about how to use various API’s to build Java applications. Chapter 12: XML and Java This chapter explains what XML is and its usage in Java.
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. how to use JDBC technology to connect with databases. This will also give an idea about J2EE application servers and their role in building enterprise applications. Java is all about writing simple programs using the built-in classes. JAR file is used to archive Java applications and is the universal way for porting Java code. This chapter will tell you some simple tricks and tips on how to use classes and also demonstrates the coding conventions and best practices that are well and truly followed in all the real world Java applications. Understanding this chapter is very important to get a good feel about what Java is all about.Chapter 6: Exception Handling This chapter explains about how to handle abnormal conditions which we call as exceptions in Java programs. Chapter 8: Threads This chapter will introduce you to concurrent processing in Java. Chapter 10: Introduction to J2EE This chapter introduces you to the world of J2EE by highlighting the important J2EE technologies and their application. This chapter also gives you a brief idea of SQL and it syntax. Understanding this chapter is very important from J2EE point of view as XML is widely used in almost all the J2EE applications. and how to execute the SQL queries against the database. Chapter 9: JAR files and Using API This chapter gives you the details about creating and using Jar files. Chapter 11: JDBC This chapter teaches you to write Java applications that talk with databases. Exception handling is one of the most important aspects of any Java based application. Java language has built-in support for writing programs that run simultaneously. This chapter explains the nuances of multi-threading and the core concepts surrounding it. This chapter will explain the details of what a database is.

the next few chapters give you all the details about Web application development using J2EE and open-source technologies. Chapter 17: Java Messaging Service (JMS) This chapter introduces you to asynchronous messaging using JMS. the Spring framework. By the end of this chapter you’ll know the basics of messaging systems and how synchronous and asynchronous messaging is implemented. It covers validator framework. Important messaging models such as Publish/Subscribe and Point-to-Point models will be discussed in detail. you’ll understand the important features of Struts and how web applications are built using it. This chapter starts with the discussion about MVC architecture. This chapter also demonstrates the usage of AOP programming. tiles framework and Internationalization. JSTL has become very popular these days and understanding JSTL tags give you an edge while building JSP pages. This chapter will explain you the key concepts like using Java Beans in JSP pages.
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. Chapter 14: JSP Programming This chapter introduces you to the most widely used web technology. By the end of this chapter. starting with this chapter. This chapter covers spring’s Core module. and building custom tags. By the end of this chapter.Chapter 13: Servlet Programming This chapter introduces you to the world of Web Applications using Java. and then explains all the important Struts components with examples. the Java Server Pages (JSP). This is an open source framework developed by Apache Software foundation. AOP module and DAO module. Chapter 15: JSTL This chapter teaches you how to build JSP pages using JSTL tags. Chapter 16: Struts This chapter introduces to the most popular and widely used web technology called Struts. Chapter 18: Spring Framework This chapter introduces you to yet another open source framework for building enterprise applications. you’ll become familiar in building web applications using JSP pages. This first chapter introduces you to basic web application development using Servlet technology and also gives you an idea about Web Containers. Since Web Applications form the basis for enterprise internet applications.

If there is a particular part of an example that I want you to pay more attention. Knowing these design patterns help you to design applications that perform well. but people who don’t have any background in Java and J2EE and looking for a jump start in their career in Java will find this book extremely useful.Chapter 19: Hibernate This chapter introduces you to one of the most popular open source ORM solution. and application development lifecycle to bring awareness about how things are handled in real world applications. Hibernate.
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. Chapter 21: Inside the Company This chapter is purely non technical and gives you a glimpse of enterprise application development lifecycle. Understanding this chapter is very important and gives you the details about how applications will be developed. This book will take you though the journey starting with core Java and all the way into J2EE and Open Source. This book also covers the non technical details of enterprise application development such as typical team structures.
Who should read this book
Java and J2EE Made Easy is for all Java developers. it will appear in bolded code font. All the examples in this book clearly demonstrate the concepts and most of the examples.
Code conventions
This book contains several examples. By the end of this chapter you’ll understand how to use hibernate framework for mapping objects to relational database systems and how to retrieve objects from database using hibernate query features. how different teams co-ordinate and all that good stuff. and can be executed without making any changes. Chapter 20: J2EE Design Patterns This chapter explains you the important J2EE design patterns that or normally used while designing enterprise applications. team collaboration. Every chapter ends with a small quiz and interview questions that should help every prospective Java developer to land in a good Job. All the examples will appear in code font. specially starting from J2EE are accompanied with step by step instructions. All the examples in this book are complete. This book is written in such a way to correlate every concept with real world scenarios.

net.
About the author
An Electrical Engineering Graduate from the University of Nebraska.j2eeconsultant.Source Code download
The complete source code for all the examples this book can be downloaded from the authors website at http://j2eeconsultant.net. point your web browser to www.
Author Online
This book has free access to web forum where you can make comments about this book and ask technical questions and receive help from the author and from other users. Currently lives in Columbus. Ohio. This page provides information about how to get to the forum. what kind of help is available. Professional activities include designing and developing software tools and training folks in Java and J2EE. To access the forum. Phani Kosuri is a Sun Certified Architect and a professional IT consultant with a handful of experience in the world of J2EE. Lincoln.
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This book was not a one man job.thanks for all their help. let me tell you this. motivation that kept me going. To my family .
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. Many of my friends played a great role in shaping out this book to what it is.acknowledgments
Firstly.thanks for giving me everything that I needed. To my friends .

Part
1
Java
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This is not good. if we write a program in a language like C or C++ or any other language. this portability feature is what makes Java distinctive from other programming languages and this is the reason why we call it as platform independent language. are targeted for a particular platform (a. Here is what makes Java distinctive from other programming languages.a operating system). then the same program cannot be executed on a different OS like. Most programming languages like C. right? Therefore. then we say the programming language as being portable in nature. What this really means is that. say Linux. then why do we need Java? Good question. C++ etc. These changes could span anywhere from modifying few lines of code to rewriting the entire program.k. And moreover. this one feature is the driving force and motivation for today’s modern enterprises that uses Java technologies. For now. we say such programming languages as being non portable in nature. this is also the reason why we are interested in learning Java. creators of Java language made it portable. Believe me. If this is the case. Sweet! We’ll see how this portability is achieved later.Chapter 1
Introduction
What is Java?
Chapter 1
Introduction
What is Java?
Java™ is a programming language introduced by Sun Microsystems Inc. say on an OS like Windows. Like any other programming languages Java is also used for building software applications.
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. Keeping the above philosophy in mind. without making some changes to the source code.
When do we say a programming language as being portable?
When the machine code generated by compiler of a programming language can be used on any OS without any modification.

Once the machine code is generated we cannot port it to a different OS and can only be used on that OS only. the question is. how does it achieve this independency? To understand. and uses the interpreter to translate bytecode to machine code. This is where Java unleashes its true power. Therefore. We all agree without any slightest hesitation that internet is part of our life. Java recognized this problem upfront and came with a novel idea by breaking the compiler into two different programs as compiler and interpreter. The advent of Java and its adaptability on internet has completely changed dimensions of business organizations and the way business is done. This is the true strength of Java.1.Chapter 1
Introduction
Java based Applications
Java based Applications
Besides Java language being platform independent. its adaptability on the Internet. there is some thing called bytecode that we need to know about.
How does Java achieve Platform independency?
First things first. Java uses compiler to translate high level Java code into bytecode. Prior to Java. This is one side of Java.
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. Internet applications Standalone applications are the applications that run on a single computer. All the command line applications and GUI based applications where funky windows are used to interact with the applications fall in this category. it is the nature of applications that we can build using Java makes it even more special. Standalone applications 2. Here is how I look at internet and Java. Java can be used to build two types of applications as listed below: 1. Internet connects people and organizations around the globe to exchange information. The other side of Java is that it can be used to build applications that can be run on the internet. As I said before that Java is platform independent. there are other languages and technologies with which internet applications are built. To achieve this. but they suffered from serious problems and limitations. This is a two step process as opposed to single step process shown in Fig 1. any language that uses just the compiler to compile as well as execute the program is platform dependent. we need to build high speed applications that run on internet. Most programming languages use compilers to translate high level program to machine code of the OS in a single step.

This bytecode can now be ported across any OS.1 Block Diagrams of single step and two step approaches.
The generated bytecode is like an intermediary code and is 100% platform independent. From Java programming standpoint. So in Java.
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. After porting it. In this two step process. the interpreter. isn't it.Chapter 1
Introduction
Single Compiler Approach
High Level Code Compiler Machine Code
Java Approach
High Level Code
Compiler
Byte Code
Interpreter
Machine Code
Fig 1. it uses javac program as a compiler and java program as interpreter.2 Demonstration of Platform Independence in Java. Smart. to convert the bytecode to machine code of that OS. Java uses the second component. we port the intermediary bytecode but not the OS specific machine code itself.2 shows the idea of platform independency. we treat the bytecode as the executable and this is how we achieve platform independence. Fig 1.
java
OS 2
Byte Code javac java OS 3
OS 1
Fig 1.

com/javase/downloads/index_jdk5.
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. The latest version of JDK at the time of writing this book is 1. we can start learning the intricacies of Java programming language. a virtual CPU that processes the Java bytecode or in other words a virtual machine that processes Java bytecode. Now that we know what Java is. we need to install Java software and set up the working environment.
Installing Java Software
Sun Microsystems Inc is the distributor for Java software which is bundled as Java Software Development Kit or simply JDK. The JDK includes both the Java compiler (javac) as well as interpreter program (java). which leads to the name Java Virtual Machine. Above all. Java is based on Object Oriented Programming model. It is rather. The examples in this book also work on Java SDK 1. it very much behaves like a CPU. we need to set up our working environment to run the examples.Chapter 1
Introduction
Java Virtual Machine (JVM)
Java Virtual Machine (JVM)
The java program. which is what we shall be using throughout. except that it is not really a physical CPU. You can however choose a different directory at the time of installation. http://java. Java can be used to build multithreaded programs. Java can be used to build Internet applications.jsp By default. Programs written in Java are highly scalable and distributable.4. which is the interpreter is also referred to as Java Virtual Machine or simply JVM because of the fact that. JDK will be installed to “C:\Program Files\j2sdk1.5.sun. Download the latest version at the following link. But before that. Java programs are portable Java programs are robust and secure.
Environment Setup
Once you downloaded and installed JDK.5”. Few reasons why we are interested in Java are: Java language is easy to understand.

Not important for us.1 JDK Directories Directory bin demo include jre Purpose This directory includes Java Compiler. let’s set up two environment variables namely JAVA_HOME and PATH. Table 1. the core libraries and security settings. This directory contains several examples.1 lists various subdirectories within JDK and their purpose Table 1.
Fig 1.5” as the home directory for Java and is referred with the name JAVA_HOME. This directory will have header files that be used by the JVM.3 below. This directory includes the Java Runtime Environment (JRE).3 JDK Directory Structure.5” directory.
Setting up the global environment variables
In this section. Interpreter (JVM) and several other utility tools. you see several sub directories as shown in Fig 1.Chapter 1
Introduction
Environment Setup
Assuming the JDK is installed to “C:\Program Files\j2sdk1.
We call the installation directory “C:\Program Files\j2sdk1. Go to Start -> Settings-> Control Panel -> System -> Advanced -> Environment Variables
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. This is one of the global environment variables we need to set up.

2. let’s create the following directory:
C:\JavaTutorial
Finally. Type JAVA_HOME for variable name and C:\Program Files\j2sdk1. Click New. and click OK. Select the PATH variable in the same section. 1. For running all the examples in this book. Type the command java. This completes the Java set up process. to validate the Java setup. Open the Command Window and move to the “JavaTraining” directory.5 as variable value.JAVA_HOME\bin to the existing variable value and click OK. 2. Click Edit. Type the command javac. 1.Chapter 1
Introduction
Environment Setup
In the system variables section do the following two things. and you should see something as shown below. and you should see something as shown below
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3. which opens a window. and append . let’s do the following three things.

java
The above command will compile the source code in the file. when I ask you to execute a program. otherwise.
How to Compile and Execute a Java Program
Let’s take a simple Java Program. to see the output as shown below:
C:\JavaTraining>java Greeting I am ready to taste Java with Passion
Note: You must not specify . This is just like a match practice before the real game.java.println(“I am ready to taste Java with Passion “). Don’t worry about what the program is.. In the next chapters. } }
Save the program as C:\JavaTraining\Greeting.class
Now. To see the bytecode file.class as shown below:
C:\ JavaTraining >dir Volume in drive C has no label. we successfully completed the Java Setup Process. we need to execute the bytecode using java command and pass the bytecode file. It will automatically figure it out. we will see that later. this is what I expect you to do. Practice the above two commands 2-3 times.out. .Chapter 1
Introduction
Environment Setup
If everything worked as above. and learn how to compile and execute it. If there are any errors it will display them. Now that we know how to compile
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. it generates the bytecode file and displays the prompt. and you will see the bytecode file Greeting. type dir command. To compile the program. 444 Greeting. Good Job. Volume Serial Number is 685A-CF9E Directory of C:\JavaTraining 10/26/2006 10/26/2006 10/26/2006 09:33 PM 09:33 PM 09:33 PM <DIR> <DIR> .class extension in the java command. use the javac command and specify the file name as shown below:
C:\JavaTraining>javac Greeting. Copy the following program into your favorite editor:
public class Greeting { public static void main (String args[]) { System.

The bytecode generated will have “.
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.Chapter 1
Introduction
Summary
and execute a program. Java uses bytecode to achieve platform independence. The program that translates the bytecode to machine code is called as Java Virtual Machine (JVM). Let’s move forward. we are ready to learn the nuances of Java Programming Language.
Summary
Java Programming language is invented by Sun Microsystems Inc. and I am sure you learned something. and to execute a program we use java command. Java is Platform Independent language. This completes this chapter.class” extension. To compile a Java program we use javac command.

C:/JavaTraining/Chapter1
From the command prompt.
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. from my experience if you know what a class is and what a method is. and some keywords associated with them. :-)
Chapter Goals
Understand the basic elements of a typical Java program Understand the primitive Java data types Learn to write and execute Java Programs Understand Class concepts Standard Java conventions and best programming practices
Environment Setup
All the programs in this chapter should be stored in the following directory. Honestly. move to the above directory. Understanding this chapter is important as it forms the basis for the next several chapters. you can call yourself as a Java Programmer.Chapter 2
Java Language Fundamentals
Chapter Goals
Chapter 2
Java Language Fundamentals
This chapter helps you to understand and write simple Java programs and highlights some of the important Java keywords. With I saying this. it doesn't mean you can skip the rest of the chapters.

Chapter 2
Java Language Fundamentals
Introduction
Introduction
This chapter introduces you to basic Java programming. One of the primary goals while designing Java is to keep things simple. the Java compiler generates bytecode for the program. State is represented by variables and behavior is represented by methods that operate on the state. This file represents the Java source code that we write. To conclude. which class is the starting point of our program? Very good question. the question that comes to our mind is. There can be one or more than one class in a single Java source file. a Java program has a class from where the execution begins. then every class must have a unique name. This is the reason why Java programs are very easy to write and understand unlike other programming languages. So.
Class
The building bock for any Java program is an entity called class. let’s start rolling the ball. So. what is a class? A class is nothing but an entity that holds state and behavior.java”. Now that we know a Java program as a collection of classes. Every class in a Java program is identified by a name called classname. If a Java file has multiple classes. The Java Virtual Machine (JVM) upon completing all the interactions with other classes finally terminates the main class. We call this class as the top-level class. A typical Java program is usually stored in a file with the extension “. This generated bytecode is stored with “. The best way to learn anything is to enjoy it.java. we can have three classes named TestClass1. there cannot be multiple top-level classes for a single Java program. Upon compiling the Java source code. As an example. everything looks unbelievably simple and beautiful. TestClass2 and TestClass3 stored in a single Java source file namely Example. So. we can say that a Java program is nothing but a collection of classes and these classes can interact with each other to process requests and return results. a Java program can only have ONE top-level class which we also call it as main class. and once you start enjoying it. Just like a flowchart has a start block from where the process starts.class” extension which we then use to execute the program and get the results.
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. Like a flow chart cannot have multiple start blocks.

let's say we have three classes namely TestClass1.class” extension.io.java is compiled using javac command. upon compilation it will still generate Test. This is how platform independence is achieved with Java. Keep life simple and easy. it’s very important to understand how a typical Java program looks like.class.class since the class name still remained the same. // One or more import statements import java. Are we not complicating things here? Complicating things is what we should always strive to avoid.util.class and TestClass3. and can be copied or ported to any operating system. TestClass2 and TestClass3 in a single Java source file Example. Good Practice: Try to write one class per Java file.class. The problem is you have to remember two file names namely SimpleTest. However. For instance. TestClass2.
A Typical Java Program
Before we start writing any Java program.java) A typical Java program.
Listing 2. then save it to a file named Test. Upon copying the class files. These class files have the so called bytecode of each class. So. you will see the same results on any platform. This way you don’t have to remember multiple filenames for the same program. Trust me.java and Test. For instance. if you save the same program to a file named SampleTest. we can then use java command on that operating system to execute the class files. if a program has a class named Test.class.java.class. take a look at a typical Java program. and the name of the bytecode file will be the name of the class with a “. import java. and ensure the Java file name is same as class name. When Example.Chapter 2
Java Language Fundamentals
Compiling a Java Program with Classes
Compiling a Java Program with Classes
When we compile a Java source file using javac command. one bytecode file will be generated for every class in the program. Note: When any Java source is compiled. This bytecode is Java specific code. it generates “. it generates three class files namely TestClass1.java.
// One Package Statement package chapter1.class” file for each and every class in the Java file.*.*. // Class Declaration public class Calculator {
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.java which upon compilation will generate Test.1 (Calculator.

if-else etc. Displaying results to the user 3. These are the text lines explaining the parts of the program. a typical Java program will have the following basic elements.5.out. In the above program. it time to learn how to write simple Java programs.
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. 2. Comments.1. Variables and Constants int i=10. The package statement is optional but using packages is a standard programming practice. without wasting any further time. Data types declarations. This is one of the goals of this book. double k = 2. The fundamental elements that are seen in any computer program are 1. Learning such things upfront will help us not only to write programs faster but also makes understanding a program much easier and fun.Chapter 2
Java Language Fundamentals
A Typical Java Program
// State.println("The sum is " + (i+k)). Though we are ready to write our first Java program at this point. let’s see what these common elements are. one or more methods void printSum(){ System. I thought it’s very useful to learn certain things that are seen frequently in any computer program written in any language. Conditional statements like if. Wherever applicable. One and only one package statement. So. A class can have ‘n’ number of variables and methods. The package statement specifies the directory in which the Java file is stored. // Behavior. It’s very important that we follow them right from the beginning to get used to them without having to spend that extra time to learn them. A class with a classname that defines the state (as variables and constants) and behavior (as methods). } }
As seen from Listing 2. the package statement denotes that the file is stored in chapter1 directory. Now that you know how a typical Java program looks like. and more importantly following best practices. I am going to highlight the best practices and standard coding conventions. One or more import statements for importing the library classes and other utility classes to use them in the current program.

int . We will not dedicate a complete chapter for the above. Like any other programming language. but we learn them as simple essentials for any program. Java also defines several primitive data types for storing data as described below. short .This data type allocates maximum of 4 bytes (32 bits) for storing the number. characters etc. This way we can save more time to learn the important things. Table 2. Control statements like while loops. it doesn’t make sense to write a program without knowing how to write the above four mentioned.This data type allocates maximum of 1 byte (8 bits) for storing the number. Table 2. long .
Data Type Declarations
Data types are the most important elements of any program as they allow us to store data.
Integer Data Types
For storing integers.Chapter 2
Java Language Fundamentals
Data Type Declarations
4. Writing a program without these is almost of no use.This data type allocates maximum of 2 bytes (16 bits) for storing the number. for loops etc. Java uses the following 4 data types based on the size of the integer data. byte .1 shows the range of values that can be safely stored in each of the above integer data types. Data types basically allow a program to store primitive data like integers. decimal numbers. In my opinion.1 Integer Data Types Data Type byte short int long Size 1 2 4 8 Range -128 to +127 -32768 to +32767 -2147483648 to 2147483647 -9223372036854775808 to + 9223372036854775807
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.This data type allocates maximum of 8 bytes (64 bits) for storing the number.

float rate = 13.
Most programs use double instead of float.
Character Data Types
For storing single characters. Unlike float.
int long byte short i k b s = = = = 10. This data types offers less precision (number of digits following the decimal point). 200. This data types offers very high precision.This data type allocates maximum of 4 bytes (32 bits) for storing the decimal number.
double rate = 2. Characters must be enclosed in single quotes during initialization as shown below:
char ch = ‘A’. Following are some examples of integer variable declarations.This data type allocates maximum of 8 bytes (64 bits) for storing the decimal numbers. This is because Java by default treats any decimal number without a literal as a double. Following is how we declare a double.
Failing use the literal will result in a compilation error.
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. 12345. 123456789. we need to use the literal ‘f’ at the end of the number as shown below.5f.
Decimal Data Types
For storing decimal numbers Java uses the following two data types based on the precision of the decimal number float . as it supports more precision.3456789. While declaring the float data type. char – This data type allocates a maximum of 2 bytes for storing characters. Java uses the following data type. we need not use any literal like ‘d’ at the end of the number. double .Chapter 2
Java Language Fundamentals
Data Type Declarations
Most Java programs often use int and long data types for storing integers.

for. So.
boolean isMarried = false. there are some simple rules to understand while doing casting. However. Just remember the previous one. the following declaration is perfectly legal. following assignments are illegal. Let’s learn what this is to complete the discussion on data types. we can convert an int data type to a float data type and vice-versa.
Casting Data types
Casting data types is nothing but converting a data type from one form to another form. Now that we know about several Java data types. Rule 1: First rule of thumb is. Following is the order of data types from smallest to largest.Chapter 2
Java Language Fundamentals
Casting Data types
In Java.
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Note that we don’t have to use single quotes or double quotes around. // No quotes with this type of representation. For instance.
Boolean Data Types
Java introduces a new data type for storing boolean values true and false as shown below. it is always illegal to assign a larger integer data type to smaller integer data type. simply ignore them.
char ch = 65. So. This is one of the most important data type and is normally used with decision making statements such as if. there is one concept around data types called datatype casting which we need to understand. Just read them once and if you think they are confusing. boolean isOwner = true. we can also store the ASCII number corresponding to the character.
The above representation allows us to do character arithmetic. You will still be fine. but this form is seldom used in programs. The numbers in the parenthesis are the sizes (in bytes) of the data types. This causes a compilation error. don’t worry about it. For instance the ASCII code of character ‘A’ is 65. while etc. byte < short < int < long Based on the above order.

the value of b will become -12. look at the following casting from int to byte. We call this as narrowing the data type. 22. This is because.
int i = 10. since it allocates 4 bytes of memory. // Explicitly convert long k to an int and then assign to i b = (byte) i. short(2) = int(4) Consider the following integer declarations. // Same reason s = (short) i. float) safely to integer data types (int. 100.
int byte short long i b s k = = = = 10.
With this assignment. We are converting larger data type to a small data type. // Illegal
To make the above assignments legal.
i = k . // Same reason
You know what. byte(1) = int(4). b = i. we need to cast them as shown below. 20. // Illegal s = i.
byte b = (byte) i. // Allocates 4 bytes for variable i
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.
With the above declarations. Consider the following integer declaration
int i = 500. As an example. Do casting only when you know for sure that the data in the larger data type is small enough to store in the smaller data type. the moral of the story is. // Illegal.even if the size of variable i is big enough to store 20. Let's see what this is with an example. right? So. long etc.
Now. Hmmm... be careful while casting data types. consider the following assignments. a single byte can only store a maximum value of 127 and JVM uses the cyclic process and makes it -12. in which case you don't loose any data.) even though the decimal data types are larger than integer data types. // i can store 500 safely.
i = (int) k.Chapter 2
Java Language Fundamentals
Casting Data types
int(4) = long(8). Rule 2: You can assign any decimal data types (double. following assignments are illegal and cause compilation errors. This is not good.. unknowingly you are paying some price to make the compiler happy and that price is data loss.

So. We know that long cannot be assigned to int variable i.
The above assignment is perfectly legal in Java and will not result in a compilation error even though a larger data type is assigned to smaller data type.5. float and double data types can be assigned to any integer data types irrespective of size.
The above is a greater than int expression and the result is long as per Rule 3(b). As an example. i = i+k. Rule 3: The result of any integer arithmetic expression is always a) An int if all the data types participating in the expression are equal to or less than int. The thumb rule is. the compiler is happy. But the price we pay is we loose the precision. Don’t believe me? Read at the following paragraph. these are seldom seen in programs. One solution is to convert the result of expression to int as shown below:
i = (int) (i+k). b) Largest data type in the expression if one of the data types participating in the expression is greater than int. lot of rules right.
k = i+k. Don't worry.
int i=10.Chapter 2
Java Language Fundamentals
Casting Data types
double d = 2.
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. // Allocates 8 bytes for variable d
With the above declarations.
i=d. // Compiler is happy. but all I say is beware of such things as they may be the root cause in a million dollar loss for a company. look at the following assignment. long k = 20.
The other solution is assigning the expression to a long variable as shown below. Hmmm.
This time we are assigning long to long. So the above assignment results in a compilation error.

what is the purpose of writing a program? So. This is more than enough.
Displaying results to user
If the program doesn’t echo results to the user. we do so as shown below. use the double quotes around text as shown below. This completes data types in Java. text messages to the console.println( "We learn Java with Passion "). double is used for storing decimal numbers. to display any results to the user (in the console). you will start to enjoy as you move forward. The result is "You are fired". Though it kind of frustrates you initially. we use the following statement.Chapter 2
Java Language Fundamentals
Displaying results to user
Let's say a customer visits a website and purchases a product for $2. ‘S’ must be in uppercase. For the rest of the chapters. In Java we usually display data in variables. 1.out. i = i*2. since it trims off the digits after decimal point. In Java. char is used for storing single characters. To display the value of a variable.
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. So. and boolean is used for storing true or false.
The above statement echoes anything specified in between the parenthesis to the console.
System.println (i). When the order is submitted. There is a trick I’ll tell you when we start writing programs.out. just remember the following things and you’ll be good. int and long are used for storing integers. if the processing system assigns the price it to an "int".com which sells millions of products a day.
System. this is my top of the list candidate. To display as text message. and this will no longer be frustrating.println(). At this point let me tell you one thing.
2. think of how many $$ it looses. If the company is like Amazon. the company looses 99 cents on the product.out. let’s see how we display them.
int i =10. Java is a case-sensitive programming language which means in the above statement.99. System.

What if you want to display more than one variable? Separate each variable with an empty text and a + operator as shown below
int i = 10. as it gives you more command on the language. int k = 20. the if block gets executed.
int i =10. we need to specify a boolean expression in the “if” statement. other wise the control goes to else block if one is present. In Java. Sometimes we need to do this.
The above statement displays the following
The Sum of 10 and 20 is 30
We use the above statement a lot in the programs.Chapter 2
Java Language Fundamentals
Conditional Statements
Simply specify the variable name in parenthesis.println ("The Sum of "+ i + " and
" + k + " is " + sum).out. let’s see the conditional operators we have in Java that return a boolean value. Following are the conditional operators that we can use: > Greater than < Less than >= Greater than Equal to <= Less then equal to == equals to != Not equals to
19
. If the expression evaluates to true.println (i + " " + k).out.
Conditional Statements
Like any programming language. Just recite the above statement five times and you’ll never forget it in your life. Before we look at the examples. int k = 20. gives you the rhythm and boosts your confidence. the conditional statements that Java uses are again the same old “if-else” blocks.
Look at even better way of displaying data. System. System. int sum = i+j.

if ( i>=10) { System. and || returns true if one of the conditions is true.
As we all know. } }
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.
int i = 10.println("i is greater than 10"). }
Example 2: The following demonstrates an if-else block. It executes all the statements within the block when the condition is evaluated to true. We can also use multiple conditions in a single statement using logical AND (&&) and OR (||) operators as shown below:
boolean test = ((10 == 10) && (5>=3)) || ( 2<5)). If the condition evaluates to false. && returns ‘true’ if both the conditions are true. } else { if ( i < 5){ System. This example also demonstrates nested ‘if’. See the following 4 examples. The above statement is a single condition statement.out.out. since the condition is evaluated to false. Look at the ‘if’ statement inside the else block.Chapter 2
Java Language Fundamentals
Conditional Statements
Any expression that uses the above operators returns a boolean value true or false as shown below
boolean test = (2 >= 5). the control goes to the else block. if (i>=10) { System.
int i = 2.println("i is less than 5"). Example 1 The following is a simple if statement.println("i is greater than 10").
The variable test will have false.out.

out. } if (found) { System. if( (i==5) || (j != 10) ){ System.out. It escaped"). int j = 20.println( "Hello 1"). while. and displays the statement ‘Found Monkey’. The first if block sets the value to true.out. } if ( (i ==10) && (j==20) ){ System. int i=10. Control statements allow a set of statements to execute repeatedly until a certain condition is met.println("Found Monkey").println("Damn.out.out. namely. if (i == 10) { found = true.Chapter 2
Java Language Fundamentals
Control Statements
Example 3: In the following example we declared a boolean variable ‘found’. There are three types of control statements. do-while and for loops.
21
. Therefore the second if block will be evaluated to true. }
Example 4: The following example demonstrates the usage of multiple conditions in if block.
int i=10. }
Control Statements
Control statements are the most important elements of any program. } if (( i==10 && j==10 )|| ( j==40) ){ System.
boolean found = false.println( "Hello 3"). It’s perfectly legal to pass a boolean variable directly to an if statement. } else { System.println( "Hello 2"). See the following four examples.

the while loop condition evaluates to false. Is this what our requirement is? Cool. // Increment or decrement } while( i<=10). and the control comes out of the loop.
int i = 1. otherwise the program hangs. Yes.out.println("Counter value is " + i).out. then the statements inside the loop will be executed. What if the condition fails for the first time itself? None of the statements inside the loop will be executed. So. it’s important that the control comes out of the loop. I have a requirement where the statements inside the loop must be executed at least once before the condition is checked. Note that variable i is declared inside
22
.Chapter 2
Java Language Fundamentals
Control Statements
Example 1:
This is an example of while loop.
int i = 1.println("Counter value is " + i). i++. At some point of time.
Example 3:
This example uses a for loop. If the condition is evaluated to true. we usually place a statement inside the loop that makes the condition fail after few iterations. Notice the semicolon after the while statement. This works much the same as previous while loop. using do-while loop is the solution. do { System.
Example 2:
This example uses a do-while loop. condition and condition changer all in one line. right? Now. we have one. Look at the next example. // Initialization while( i<= 10) { System. // Increment or decrement }
Now. except the statements in the loop will be executed before the condition is checked. This will increment the value in variable i by 1 every time the control comes inside the loop. what should I do? Is there is simpler solution. So. In the following code it is the i++ statement. The condition is specified in the while statement. When the value of i becomes 11. So. a small question for you. i++. This is the simplest of all in the sense it declares initializer. we are 100% guaranteed that the statements in the loop are executed at least once.

This saves us whole lot of precious time.2 shows a single class defined in the Java file. We finished learning the basic elements of any Java program and we are now ready to take on the big ones.i++) { for ( int k = 1. you need not declare the variables at the beginning of the program. the inner one loops 10 times. i<=10. Unlike in C.out. }
Example 4:
We can also define a loop within a loop as shown below.Chapter 2
Java Language Fundamentals
First Java Program
the loop itself. k++) { System.
for (int i =1. Listing 2. The standard syntax for defining a class is shown below. You are free to declare them wherever you want.
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. one of the coolest features in Java.out. k<= 10. please follow the steps outlined in the environment setup at the beginning of the chapter.
First Java Program
As I said before.println("Count is " + (i+k)). This is perfectly legal. } }
That’s it guys.println("Count value is " + i). Declare and use.
class <class name> { }
class is a Java keyword that identifies a Java class. So.i++) { System. To run all the examples in this chapter. The reason I’ve explained all this upfront before even writing a single Java program is that you will feel more comfortable and confident when you start writing real programs. These are called as nested loops. guess the output. a Java file simply contains one or more classes. i<=10.
for (int i =1. For every value of i in the outer loop.

3 (BasicClass. This class can be saved to a file with the name of your choice.java’ file and we execute the ‘.java
The above command generates TestClass.
The reason JVM has resulted in the above error message is due to the fact that the class doesn’t have anything defined in it.
class TestClass{ }
As you can see from the above code. Listing 2.2 (SimpleClass.java) Java source with more than one class. Compile the program as shown below:
C:/>JavaTraining>Chapter1>javac SimpleClass. Once again all the classes are empty.
class BasicClass11 { } class BasicClass12 { } class BasicClass13 { }
The above Java source file has three classes namely BasicClass11. In Java. the name of the class is TestClass. BasicClass12 and BasicClass13 which are saved to the file named BasicClass.
C:/>JavaTraining>Chapter1>java TestClass
The above command produces the following output. Therefore. we always compile the ‘. To execute the program.java. Let's look at next program.java) A simple class.Chapter 2
Java Language Fundamentals
First Java Program
Listing 2. we need to pass the name of the classfile without the extension as shown below. In this case.class’ file to see the results. the program is of no use.
Error: Class has no main method.java.java
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. Compile the program as shown below:
C:/>JavaTraining>Chapter1>javac BasicClass.class file since the Java source defined a class with the name TestClass. it is saved in the file named SimpleClass.

and you know what. To execute any class. Ok. which class should we execute? You can try executing all the three classes as shown below. since all the classes are empty.
C:/>JavaTraining>Chapter1>java BasicClass11 Error: Main method not found C:/>JavaTraining>Chapter1>java BasicClass12 Error: Main method not found C:/>JavaTraining>Chapter1>java BasicClass13 Error: Main method not found
Now.java) A simple class.class. our goal is to make all the above classes executable.4. right? We can’t blame the JVM. If we compile and execute the above program like we did in the previous example.
class BasicClass11 { public static void main (String args[]) { } } class BasicClass12 { public static void main (String args[]) { } } class BasicClass13 { public static void main (String args[]) { } }
This time our program added a main method in each of the classes.class. the error will disappear. So. they all give you the same error “Main method not found”.
public static void main( String args[] ){ }
25
. The resulting three class files will be BasicClass11. Listing 2. let’s add that method in all the classes and see what happens. Since JVM complained about the absence of main method.4 (BasicClass.class and BasicClass13.Chapter 2
Java Language Fundamentals
First Java Program
Since we defined three classes. and the main method must be exactly as shown below. Modify the above classes as shown in Listing 2. BasicClass12. This error makes sense. one for each. here is the rule. Java compiler generates three class files. Now the question is. the class must have a main method.

the rule is. and if it is the case then Java source file name must be the name of the public class name. we should obey the JVM order and sincerely rename the file to BasicClass11. Good Practice: A class name should always start with uppercase character. and every word in class name should also start with uppercase letter. Add the keyword public to BasicClass11 class as shown below. The reason is simple. the file name must be the class name of the top-level class. JVM treats it as top level class. Always write one class per Java file. So. So.Chapter 2
Java Language Fundamentals
First Java Program
At this point of time. To your surprise. to keep things simple and clean. and without any surprises the error disappears. then the file name must be same as the public class name. Save the file. You can’t. Can more than one class in a single Java file be declared as public? Strictly No. HttpServlet. You cannot have two filenames for the same file. void and all that. Therefore for the above program to compile. Connection etc. the class must define the main method. and compile it again. it’s not a good practice to write more than one class in a single java file. if a Java file has a class declared as public. I am going to throw an error”. “Hey buddy. As simple as that.
26
.. I am going to write just one public class in one Java file with or without the main method. EntityContext. Let me tell you one thing here. static.
public class BasicClass11
Now save the program and recompile it again. You’ll know about these later when time comes.
C:/>JavaTraining>Chapter1>javac BasicClass. Let’s do a small experiment with the above code. All we need to know is. In essence. if your code has a top level class. a single Java file can only have one public class. to execute a Java class. Otherwise. Let me now ask you a question here. don’t worry about the words public. from here onwards. Though we executed the classes successfully without any errors. InitialContext. and says. Some example class names are shown below HelloWorld. you’ll see the following compilation error.java Error: The file name must be the public class BasicClass11
The moment we added public keyword to BasicClass11 class.java.

public class Greeting { {
public static void main ( String args[]) } }
We finally reached a point where.5.java) A simple class that echoes a greeting. Listing 2. and 2. learning the basic programming elements upfront makes it easy to write and understand them pretty fast. our Java program would look like as shown in Listing 2.
Main method of class
This is the standard method that the JVM looks in a class to start executing the statements. 1. Listing 2. So. With the above good practices and naming conventions. i<=5.Chapter 2
Java Language Fundamentals
Main method of class
The above class naming convention is universal standard in Java.6 is a simple program that uses for loop that repeats five times. the main method is the starting point of execution of any Java program like a start block in a flowchart.out. So the message gets printed five times in the console. i++){ System.println ( "Keep things Simple").
27
. Compile and execute the program as shown below. what a System. See. } } }
Listing 2.6 (Message.
public class Message {
public static void main ( String args[]) { for (int i=1. Write a main method in the class. So. so let’s follow it.5 (Greeting. Very good. That’s it.out statement is and how they work. to compile and execute a Java program without any errors. We should write a public class in the Java file. We don’t have to scratch our minds as to what a for loop is. let’s start writing real productive programs.java) A simple class.

java C:/>JavaTraining>Chapter1>java Message
The output of the program is
Keep Keep Keep Keep Keep things things things things things Simple Simple Simple Simple Simple
Listing 2.
C:/>JavaTraining>Chapter1>javac SumSquare. and we print the value in the ‘sum’ variable. i++) { sum = sum + i * i. for (int i=1. let’s write some modular programs using methods. I expect you to a) Copy the file into the “chapter1” directory and b) Execute javac and java commands like you did until now. Copy the above program to the following directory to compile and execute the program.println(“The sum of squares of first 30 numbers is “ + sum ).
28
.out. From here onwards. So. } }
The above program again uses for loop and computes the sum of squares of certain numbers. when I ask you to compile and execute a program. i<=30.
public class SumSquare { {
public static void main ( String args[]) long sum = 0.Chapter 2
Java Language Fundamentals
Main method of class
C:/>JavaTraining>Chapter1>javac Message.java) A simple class that computes the sum of squares. When the value of variable i becomes 31.java C:/>JavaTraining>Chapter1>java SumSquare
The result of the above program is
The sum of squares of first 30 numbers is 9455
I know the above two programs are not so interesting.7 (SumSquare.
} System. the loop terminates.

Following is how the program works. Main then calls sayHi() method. } }
The example in Listing 2.8 (TaxPayer.java) A simple class that uses methods. 1. which prints a message and returns back. However this is usually not the case. Moreover.println("I am in main method"). This example introduces you to methods in a class. 5. As I said before.println("I am in sayHi method"). Main method prints some message. sayHi().out. methods can call each other to generate results. a class can have any number of methods besides the main method. let’s see how to define a method in a class.out.out. } static void payTaxes() { System. // Call methods from here payTaxes(). All these methods define statements to compute the results.println("Thanks for Paying taxes"). right? Compile and execute the program only to see the following result. payTaxes() method prints another message and returns back to main. main method terminates. Since there are no more statements to execute. our class just had the main method with all the statements.Chapter 2
Java Language Fundamentals
Main method of class
In all the previous examples. and then calls the method payTaxes() method.8 is more interesting and spicy. 4. Program execution begins in the main method 2. Listing 2.
I am in main method Thanks for Paying taxes I am in sayHi method
29
. 3. A class can have one or more methods which define the behavior of the class. } static void sayHi() { System. So. A method is like a reusable module with in a class that can be invoked any number of times.
public class TaxPayer { public static void main(String args[]) { System. Any doubts? No.

Chapter 2
Java Language Fundamentals
Class Methods
Before we start writing further examples. Need to specify the type of the data being returned as the return type. they will be treated as object level properties. following is how the method should look like:
static int getAmount(){ int amount = 1000. let’s learn some details about methods and their intricacies. we use static in every method declaration. which is the class. we need set the return type as void as shown below
static void sayHello() { }
If the method does return some data back. Don’t even think about objects for now. otherwise. If the method or a variable is declared as static. 1. For instance. The return type of a method denotes the type of data being returned back to whoever called the method.
Class Methods
The syntax for writing a method in a class is. Right now we are only dealing with the first entity. In Java. variables from object level methods. <one or more keywords> <return type> methodName (argument list) { } One of the keywords we need to use at least for now is static. there are only two fundamental entities called class and object. Ok. Why? Good question. Otherwise. it needs to do two things. Return the data using the keyword return. variables we use the keyword static. 2. return amount. }
It is very important that the data returned match with the declared return type. Make sense? Since we are talking about class methods and not object level methods. If the method doesn’t return any data back. Therefore. We’ll see them later.
30
. it results in a compilation error. to distinguish class level methods. if a method returns a number of type int. then they will be treated as class level properties. Both class and object can have methods as well as variables.

Let’s implement a search functionality that allows searching for a resource on Internet and directory. the ‘argument list’ will be empty within the parenthesis as shown in the above two examples. */
The examples we are going to write from here inwards are more interesting. we can write single line comments and multi line comments as shown below:
// This is a single line comment // This is another comment /* The following code convert a given string into a number and inserts into the database. As an example. These variables are declared within the parenthesis in the method declaration. the numbers will be stored in variables a and b. Instead of writing all the functionality in one class. This is one of the best practices. so that we can reuse the classes. then we need to specify the type of data that it takes by declaring the variable types.Chapter 2
Java Language Fundamentals
Passing Arguments to Methods
Passing Arguments to Methods
Method arguments are nothing but a set of variables a method uses to take the data for it to process and return results. which can then be used within the method body for processing. If you notice. If the method doesn’t take any data. This is all about methods you need to know at this point. This is how we need to write programs. We will get away from this approach and start writing code in multiple classes and have them call each other.
31
. if the method does take arguments. the method declaration would look like as shown below:
public void setData(int a. However. in all the examples thus far we just used one class to define all the methods and invoked them from the main method. double b) { }
When the invoker of this method sends two numbers. try logically distributing the code into multiple classes. if the method takes an integer and a double argument. In Java.
Comments
Using comments in program is very good practice as it improves the readability of the program.

java) Class to search on directory. otherwise their existence doesn’t make any sense.out. both the search engine classes do not have a main method defined.8a (InternetSearchEngine.8b (DirectorySearchEngine. we will write another class named PremierSearch.out.8 a & b.println("Found 20 results on Web"). right? Therefore. } }
Listing 2.
public class DirectorySearchEngine { static void search() { System. which will use the above classes to search for resources.println("Found 10 results on Novel Directory").out.Chapter 2
Java Language Fundamentals
Passing Arguments to Methods
Since we need to search for resources in two different places. let’s write two classes namely InternetSearchEngine.java) Class to search on internet and directory. Listing 2.8c has the implementation for this class.java and define a search()method in each. DirectorySearchEngine.java and DirectorySearchEngine.println(" Search Completed "). but cannot execute them. System.
32
. Listing 2. } }
Look at the above two search classes in Listing 2.8c (PremierSearch. Listing 2. So you can compile them. These are like utility classes that some other class must use them.search().
public class InternetSearchEngine { static void search() { System.
public class PremierSearch { public static void main(String args[]) { // Local Search search().search(). // Other searches InternetSearchEngine.java) Class to search on internet. If you noticed.

We used the class name to invoke the methods in different classes. But if I wanted to say the color of other person shirt. To get around with this.Chapter 2
Java Language Fundamentals
Passing Arguments to Methods
} static void search() { System. } }
The PremierSearch class has its own search() method for local search. However. Note: You need to compile all three classes using javac command and then execute the PremierSearch class to see the results. it needs to prefix the class name with a dot before the method name. it must use the following syntax: ClassName. Compile and execute the classes as shown below:
C:/>JavaTraining>Chapter1>javac InternetSearchEngine. Writing functionality in multiple classes allows loose coupling and at the same time code reusability.
Found 5 results in Local Found 20 results on Web Found 10 results in Novel Directory Search Completed
Trick: I know it’s tedious to compile each and every program using the javac command shown above. Above is exactly what we did in PremierSearch class. I must say "The color of Tom’s shirt is red". See.println("Found 5 results in Local"). if you want to add new search functionality. use the following simplified command to compile all java files in one step (using *). The rule is. it probably refers to my shirt. for a class to invoke static methods in another class. you can write a new class with a search method.out. Tomorrow.
33
. Java is closely related with how we talk every day.java C:/>JavaTraining>Chapter1>javac DirectorySearchEngine. It calls this method by just specifying the method name like we did in previous examples. the color of the shirt is red.methodName().java C:/>JavaTraining>Chapter1>javac PremierSearch.java C:/>JavaTraining>Chapter1>java PremierSearch
The result of the above program will be. If I say. I have to use his name. and then call it from where ever you want. to call the search() methods in the two search engine classes. Isn’t this nice? I am sure you are with me.

} }
Now look at the LoanCalculatorTest class. which is the approved amount. // Get the results double amount = LoanCalculator. and returns the approved amount based on some conditions. the return type is set to double as shown below:
static double getApprovedAmount()
Listing 2. and execute the test class to see the following results. It simply invokes the appropriate methods on the calculator class by giving the details and taking the essentials. Whenever we define variables in a class. This implies that. the getApprovedAmount() method reads the data from the populated global variables. Also note that this method has a return statement at the end to return the approved amount which we declared as double. Note this point. in its method declaration.java) Test class that passes the details and take the essentials.
public class LoanCalculatorTest { public static void main(String args[]) { // Pass the details LoanCalculator.getApprovedAmount().Chapter 2
Java Language Fundamentals
Passing Arguments to Methods
The LoanCalculator class has defined three variables and three methods. Another important thing is that the class level global variables must be declared static.
Approved Amount = $1000.setCreditScore(650). they can be accessed or shared by all the methods of that class.00).0
This is how we need to define variables and methods in Java program. static methods can access static global variables only. We call them as global variables of the class. System.
35
. Finally.println("Approved Amount = $" + amount). Therefore. Similarly the other set methods. LoanCalculator. Let’s look at next concept. Compile the two classes.9b (LoanCalculatorTest. The setCreditScore() method takes the credit score and stores it in creditScore variable.setAnnualIncome(55000.out.

A static block looks as shown below:
static { // Initialization Statements }
The purpose of static block in a class is for class level initialization. it’s very important to initialize some of its internal components. a static block. More often than not. we require JVM to perform some initialization before it starts executing the main method.10 (StaticBlockDemo. } public static void main(String args[]) { System.out. However.println("Remaining 60% Class is also Initialized"). and the most important application of static. } static { System. Listing 2. before we use something like a database.out. it’s not a good practice to use multiple static blocks. and the JVM will execute all of them one by one. For instance. Initialization is almost a must for any software application.10 demonstrates the usage of static block.
public class StaticBlockDemo { static { System.out. Listing 2.java) Class demonstrating the usage of static block. They allow Java programs to execute certain initialization statements before the actual execution begins.Chapter 2
Java Language Fundamentals
Static Blocks
Static Blocks
So far we used static keyword in a class with methods and variables only. } }
36
. This is where static blocks come in handy. Is there another usage of static that we missed? Yes there is. A static block has the following properties.println("I am in Main"). It has no name It gets invoked before the main method is invoked JVM will only invoke it once There can be more than one static blocks in a class.println("40% Class is Initialized").

For all the cases.out. Listing 2. Case 1: See listing 2. and the above mentioned properties of static block will make sense.println("Database Initialized"). At the same time we must not initialize the database every time we get a connection. The program is very simple. The result of the program will be. We will see three cases on this example from bad to good to great.java) Version 1 implementation of the database class
public class Database { static void initialize() { // Do the initialization here System. 2. let’s look at a classical database example with and without a static block. The above example is pretty simple and doesn’t really demonstrate the beauty of static blocks. In this example. We cannot get a connection without initializing the database. We should not call the initialize method from outside database class.11a for the version of database class.11a (Database. let’s implement a database functionality where the database returns two connections to the requestor. Only Database should initialize itself.Chapter 2
Java Language Fundamentals
Static Blocks
When the JVM executes the above class. the test class remains the same. and you'll notice its importance. it first invokes all the static blocks one by one before it begins executing the main method. Challenges 1. but we will enhance the Database class to come up with a clean and better solution. We will simply write a class named Database with two methods namely initialize() and getConnection() methods.
40% Class is Initialized Remaining 60% Class is also Initialized I am in Main
Use static blocks whenever you want to execute certain statements prior to the execution of main method. Initialization must be taken care by the database class itself. The test class is not allowed to invoke the initialize() method as it is internal to database. We will then write a test class that only invokes the getConnection() method twice to retrieve two connections.
37
. Compile and execute the program. So.

but is safe. Usually. ensuring that the database is always initialized before returning the connection.11b (DatabaseTest.
38
. } }
If you look at the above test class.
public class DatabaseTest { public static void main(String args[]) { // Get teo connections. this version of database class initialized itself twice. it must be done as few times as possible. This implementation is not at all good because it is misusing the resources. Database. It bluntly invokes the initialize() method every time the getConnection() method is invoked. Listing 2. it invokes the getConnection() method twice to get two connections from the database to meet the requirement. This is exactly what we want to do. Database. Listing 2. to get two connections successfully. Compile the above two classes and execute the test class to see the following result.getConnection().Chapter 2
Java Language Fundamentals
Static Blocks
} static void getConnection() { // Calls the initialization method initialize().java) Test class for retrieving the connections.11b shows the test class that tries to get the two connections. System. Let’s see if we can modify the database class to cut down the number of initializations.getConnection(). This is therefore a very bad implementation.println("Connection established").out. database initialization is a costly operation and consumes significant amount of resources.
Database Initialized Connection established Database Initialized Connection established
As you can see from the above code. } }
This class is not smart. Therefore.

Case 3: In the previous example. right? With this updated class.Chapter 2
Java Language Fundamentals
Static Blocks
Case 2: In this case. you might be wondering that we were able to solve the problem in second case itself. } System. it updates the variable val to 1. when the test class invokes the connection method for the first time. it introduced some additional logic around the call to initialize() method in the getConnection() method.11c (Database. Look at the following enhanced version of the database class in listing 2. static void initialize() { System.11c. } static void getConnection() { if (val == 0) { initialize(). To ensure that the database is initialized only once.java) Version 2 implementation of the database class
public class Database { static int val = 0. Listing 2. At this point. so why do we need a third case.out. What I don't like in the second case is the introduction of additional
39
. } }
Look at the above enhanced Database class. Smart logic. and the control goes inside the if block and invokes the initialize() method. if you execute the same test class. the variable val will be 0. the database class is good. After successful initialization within the method. the condition fails and will not get initialized.println("Database Initialized").out. we will tweak the above database class to ensure that it is initialized just once. val = 1. The way the logic works is. In this case.println("Connection established"). we didn't change anything in the test class. the output will be what we wanted it to be as shown below:
Database Initialized Connection established Connection established. For the second call to the method from the test class. All we did is tweaked the database class to get the right output.

go through them again. Always remember one thing.println("Database Initialized").java) Version 3 implementation of the database class
public class Database { static { System. Following is how it works.println("Connection established"). and directly invokes the method. and then invokes the connection method. and you’ll be fine.Chapter 2
Java Language Fundamentals
Static Blocks
logic to meet the requirement.out. JVM bypasses the static block as it already executed before. JVM invokes the static block.out. when the program is getting complicated. JVM ensures that it calls this block only once.
40
. This is the best solution we can get. So try to make the program as simple as possible with fewer statements. we simply put all the initialization statements in this block. And also remember the fact that. By doing this. This is all what we need to know about classes. and moves the initialization statements into a static block as shown in listing 2. We have seen several examples and I am sure we learned a lot in no time. Execute the test class again.11d (Database. My third case is smart and classical. Thumb Rule: Put all the class level initialization statements in the static block. every additional line code is a potential bug. It permanently removes the initialize() method in the database class.. If some things are confusing. the results are same. when the getConnection() method is called for the first time from the test class. i. When the control comes for the first time to this class. Yet. But notice how simple the database class is. } static void getConnection() { System. you are always going in the wrong direction. As we know that the static block gets invoked only once. From the second call onwards to the connection method.11d. No variables. no conditions and no initialization method. and you’ll be surprised to see the same result as Case 2.e. Listing 2. } }
Use the above database class and compile the program. right? Simple and classical.

A class can have any number of static blocks. static{
41
. Static methods can only access static global variables.Chapter 2
Java Language Fundamentals
Summary
Summary
A class is the building block of Java program. it must have a main method.methodName syntax. JVM invokes the static blocks before invoking the main method. To execute a class. Given a class DriverManager with a method getConnection. 4.
Time to play 50-50
1. A class will have methods and variables. but it’s a good practice to use only one. which of the following is valid way of invoking the method a) DriverManager.getConnection(). Which of the following is a valid main method? a) public void static main(String str[]) b) public static void main(String str[]) 2.getConnection b) DriverManager. A class uses static blocks for initialization purposes. it must use ClassName. Several classes can interact with other. For one class to invoke static methods in different class. methods and Object level variables and methods. What is the output of the following program
public class InitialContext { static int i. Which of the following is a valid class level method a) double getAmount() b) static double getAmount() 3. Class level methods and variables must be declared as static. A class can have Class-level variables.

Answer: It has no name It gets invoked before the main method is invoked JVM will only invoke it once There can be more than one static blocks in a class.out. Let’s move forward.println( “ Context Initialized “). and the JVM will execute all of them one by one. System. } public static void main( String args[]){ System. } }
a) Context Initialized 10 b) Context Initialized 0 5. it’s not a good practice to use multiple static blocks.Chapter 2
Java Language Fundamentals
Summary
i = 10. Congratulations! You have successfully completed Chapter 1.
Interview Questions
Question: List the properties of a static block.out.
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. However. b) int k = 5 * 5.println (i). Which of the following are valid assignments a) byte b = 5 * 5.

move to the above directory.
Chapter Goals
Understand the notion of packages Understand the keywords associated with packages
Environment Setup
All the programs in this chapter should be stored in the following directory.
43
.Chapter 3
Packages
Chapter Goals
Chapter 3
Packages
By the end of this chapter you will understand the notion of packages in Java. This chapter demonstrates the usage of various keywords associated with packages and how to implement access control for classes within packages.
C:/JavaTraining/Chapter3
From the command prompt.

What is a Package?
A package is a collection of classes that allows organizing the code effectively and efficiently without any namespace collisions. It would be much better if we store them in separate directories.
Creating and Declaring Packages
Creating a package is nothing but creating a directory on hard drive. Java packages is really not a concept. The good thing with using packages is that besides organizing classes. It hardly takes couple minutes to understand what packages are and how to use them. server related classes. Once we have this directory (from here onwards I am going to refer this as package) created. utility classes and many more. Consider for instance we are developing a chatting application. You really don’t have to concentrate seriously to understand this chapter. A typical Java application comprises of several Java classes and organizing the classes in a systematic fashion is very important from maintenance standpoint.Chapter 3
Packages
Introduction
Introduction
This chapter introduces you to the notion of packages in Java. This is very much like organizing files on your computer system. Such application contains several classes that can be categorized as UI related classes. Let’s create a package named src in our JavaTraining/Chapter3 directory as shown below:
C:/JavaTraining/Chapter3/src
44
. Without wasting any time let’s quickly get though all the important things we need to know about packages. there is one statement we need to add at the beginning of source code which tells the JVM that the class indeed belongs to that package. we can store all the classes in this package. I am going to keep this chapter really short by giving you the important details about packages. A package is nothing but a directory in which we store Java classes. but we can think of it as a good practice. However. FTP classes. Storing all these classes in just one directory poses several maintenance problems. This is what Java packages is all about. we can also provide some level of security to the classes which comes automatically.

java) Class using packages
// Package statement package src. You can store any number of classes in a single package.Chapter 3
Packages
Creating and Declaring Packages
If we wish to store a class in the above package. what is a root directory? A root directory is nothing by the directory of your project. fundamentally. Before we start using the packages. we can create another package named examples within src package.
There is nothing wrong in your thinking. For instance. The syntax for declaring the package is. However.println("Hello World"). the code will look as shown in listing 3. So. Let’s say we are developing a financial application. the first line is the package statements that tells the JVM that this class strictly belongs to the package src. it doesn’t work this way since the package names are relative names.examples. Moreover.1. 1. like we create directories within directories we can also have packages within packages. } }
As you can see from the code. All the packages we create must be created within the root directory.out. There are two simple rules to keep in mind though. public class HelloWorld{ public static void main(String args[]){ System.1 (HelloWorld. The keyword we use for package declaration is package. The package statement must be the first statement in the program. A class can only have ONE package statement 2. So. package <packagename>. Listing 3.
Surprised? May be you thought the package statement should instead be
package examples. we first need to fix the root directory. The classes within examples package will then use the following package statement:
package src. we need to
45
.

java StringUtils. you might want to have one or more Java files with different functionality but share the same name. Package Statement
SystemLog.
Namespace collisions
More often than not. all the package names are now with respect to the root directory “RetailBanking”.java package core. Table 3.java package core. and other date class for processing database related dates.java FileLogger.java I am sure the above table made things clear.
RetailBanking core Calcuator. You cannot have two files with the same name in the same directory.1. Let’s say we have created the following directory for this application. Rather than using two class names like
46
. The reason is simple. For instance. Consider the following package structure for organizing our classes.java logging SystemLog.java utils DateFormatter.utils.1 Class and Package names Class Name Calcuator.java package core.logging.java DateFormatter. As you can see.
C:/RetailBanking
The above directory will now be the root directory for all the components within the application. Once created. the package names for various classes will be as shown in table 3. the package names should be with respect to the root directory. All the packages we create will be created from this directory onwards for storing the class files. This would be impossible without packages. StringUtils.Chapter 3
Packages
Creating and Declaring Packages
choose a directory for storing all our work. FileLogger. I can have one date class for processing general purpose dates.java
Based the above package structure.

some other classes from some other company. <companyName>.xlog.Date and database.Chapter 3
Packages
Creating and Declaring Packages
RegularDate and DatabaseDate.abc. The fully qualified name will always be unique.logging org.etools. XLOG.
Package Naming Conventions
Although you can have any arbitrary names for package names. then your application packaging structure can be.Date are the fully qualified names of the two date classes. if you own a company named “eTools Inc”.<module name> The type of company is normally commercial or organization in which cases we use either com or org.
com.java database Date.general and so on….
com.etools. core. I can use same name for both the files but should store them in two different packages as shown below:
core Date. All these companies follow a standard naming convention as shown below: <type of company>.calcualtors. For instance in the above example. we always follow standard naming conventions.web com. we can use the logging classes from company say. Following are some of package names using the above convention.
47
.xml
Using the above conventions.
Fully Qualified Name of a Class
A fully qualified name of the class is the class name along with the package name. which sells commercial tools.java
The first one uses the package statement with the name core and the second one with the name database.calculators com. For instance. A typical Java application uses several third party classes from different companies.zee.

Unlike a package statement.PrintWriter.2 (Calculator. a Java program can have any number of import statements as shown below:
import java. As an example. i <= 100.
The asterisk (*) at the end indicates all the classes within the package.2.java) Class using packages
package examples. let’s consider the code in listing 3.out.println(sum).io.lang java. i++) { sum = sum + i. However.Chapter 3
Packages
Java Core Libraries
Java Core Libraries
Java comes with a rich set of library classes and all these classes are very cleanly organized in the form of packages. we need to import them using the import keyword as shown below:
import java. } }
48
.
import java.util. } System.
Compiling and executing Programs
There is a standard way of compiling and executing a class that uses packages. To use the classes stored in above packages.* .*. To better understand.io. Listing 3. public class Calculator { public static void main(String args[]) { int sum = 0.lang.*. if you just want to import one specific class in a particular package. following are the some of the important packages in the Java core library:
java. import java. you can do so as.util
All the above packages will have several classes. Our Java programs simply need to import these library classes by specifying the package names. for (int i = 0.

One of the important features of packages is that when used in programs. JVM will provide some level of security to classes within packages. “chapter3” is the root directory. it is very useful from application point of view.) instead of slash (\) and without any extension.java
Once all the programs are compiled. If we want to compile all the classes in the same package. we need to use dot (. use and import packages. Java provides three keywords namely public. it’s time to understand the most important aspect of packages which is access control. A Java application is all about invoking the methods in classes that are distributed in several packages. Can a class A in one package access a class B in different package?
49
. The javac command should specify the package name and the class name separated by a “\”. we can use the following syntax
C:/JavaTraining/chapter3> javac examples\*. Based on this. Using these three keywords and together with packages.java
In the above command. Now that you know how to create. The above program should be compiled as shown below:
C:/JavaTraining/chapter3> javac examples\Calculator. Though this level of security doesn’t protect the code from hackers. following are the important questions we can ask ourselves. the programs should always be compiled from the root directory only.
Access Control using Packages
So far we’ve seen how to define a class using packages. We call this as access control. protected and private which we call them as access modifiers. we then have to execute the main class using the following command:
C:/JavaTraining/chapter3> java examples. This is how you need to compile and execute the classes using packages. classes and methods can be implemented with restricted access. 1.Chapter 3
Packages
Access Control using Packages
When ever we use packages. Let’s see what this is all about.Calculator
Note that while executing the program.

Protected Access
This keyword is used with methods and variables. Table 3. Also.2 lists which access modifiers are applicable to which entities.1 Class and Package names public Yes Yes Yes protected No Yes Yes private No Yes Yes default Yes Yes Yes
class method variable
As you can see from the above table. The same is applicable to variables and methods if they are declared public. all the access modifier keywords can be used with classes. protected int k=20. ( Public Method) protected void sayHello(). it is important to note that “default” means no keyword. Access modifiers are used with classes. One good example is the utility classes which should always be declared as public since they are normally shared by all the remaining classes within the application irrespective of package they are present in. methods and variables. public int k = 20. methods and variables and is the most generous one. methods and variables except that we don’t use private and protected with a class. Any class will be able to access them. If used with a class. If a method or variable is
50
. Following are some of the examples:
public class Test class Test public void sayHello(). The answer is “yes” if we use public keyword and “no” if we use private keyword. ( Private Method) void sayHello(). ( Default Method) private int i=10. The scope of this keyword is almost close to public keyword with just one small access restriction.Chapter 3
Packages
Access Control using Packages
2.
Public Access
This keyword is used with classes. can a class A be able to access class B methods? The answers to both the questions are Yes and No. will allow classes in the same package or in any other package access it without any restriction. Table 3. Even if the answer is “yes” for the above question. ( Protected Method) private void sayHello().

51
. I will explain them in detail when we start using them. only classes within the same package and “child” classes within different package can access them. Packages eliminate namespace collisions. The private methods as the name suggests are private to that class. all the programs are based on ‘Structured and Modular Programming models’. and cannot be shared with other classes.Chapter 3
Packages
Summary
declared protected. This is all you need to know about access modifiers and packages. When used. if you noticed carefully. We’ll see them later. With this brief overview about packages. we always follow good practices. The next chapter is the most and most important one. an Object. method or variable. then we say that they all have default access. Since we are aspiring to become a professional developer. it still supports the old programming models. Don’t worry about what child classes are for now. A class must use the package keyword to specify the package a class belongs to. Using proper access modifiers and packages is only a good practice but not a “must”. let’s move on with the rest of the chapters. A class typically has a combination of public and private methods. This is the lowest level of access we can provide to any entity. This means only the classes in the same package can access them. Don’t worry about them for now. as it introduces you to the second type of entity in Java. the order of access from highest to lowest is public ->protected->default->private From the next chapters onwards we will start using these access modifier keywords. From the above discussion.
Private Access
This keyword is only used with methods and variables. they are only limited to the class in which they are present even if the class is public. You know what. Though Java’s strength is ‘Object Oriented Programming’ which we will see in the next chapter. Sweet!
Summary
Packages are used for organizing the classes in different directories.
Default Access
If none of the access modifiers are used with a class.

util. True/False 3. Which of the following access modifier should be used to allow access in the same package a) public b) default 6.util package a) package java. a) Package b) package 2.Hashtable.
Time to play 50-50
1. For a class to use such library classes. Using packages provides access control to classes.Chapter 3
Packages
Summary
A class can only have one package statement and it must be the first statement in the code. All the library classes are organized in several packages. True/False 4. Which of the following access modifier is used with a method to prevent external classes from accessing it? a) protected b) private
52
. The keywords that support in access control are public. Unlike a package statement. protected and private The order of access modifiers from highest to lowest is public. a class can have any number of import statements.util.Hashtable. it needs to import them using the import statement. protected. default and private. Which is the following keyword is used to specify a package. A class can have more then one import statements. Which of the following statement imports a class named Hashtable in the java. A class can have more than one package statement. b) import java. 5.

or a method or a variable. When a method or a variable is declared protected.
53
. Private keyword is used only with methods and variables. When none of the keywords are used with a class. Let’s move on. Usually a class will not be declared as protected. These can only be accessed only within the class that contains them. Congratulations! You’ve successfully completed this chapter and I am sure you are ready to move to the next chapter. it is given a default access. protected and private Question: Explain the differences between public. they can be accessed by all the classes in the same package and child classes in different packages. This means they can only be accessed by classes in the same package.Chapter 3
Packages
Interview Questions
Interview Questions
Question: What are the different access modifiers in Java? Answer: public. protected and private keywords Answer: When a class or a method or a variable is declared public. they can be accessed by any class in any package. This is the smallest level of access.

move to the following root directory and get ready.
Chapter Goals
Understand the difference between Class and Object Learn how to create Objects from Classes Learn to define interaction between objects Understand Object Oriented concepts like Encapsulation. Understand the notion of Constructors and some important keywords Understand Abstract classes and Interfaces. You'll surely enjoy this chapter since OOP is fun and its associated terminology is heard in our day to day life. Inheritance etc. you will know how to build Object Oriented Java applications.Chapter 4
Object Oriented Programming
Chapter Goals
Chapter 4
Object Oriented Programming
By the end of this chapter.
C:/JavaTraining/Chapter4
54
.
C:/JavaTraining/Chapter4/objectconcepts objectconcepts is a package in which we will store the programs. To compile the
programs.
Environment Setup
All the programs in this chapter should be stored in the following directory. This chapter will demonstrate Object Oriented Programming (OOP) concepts and their implementation in Java.

We like keeping things simple. loosely coupled and flexible applications unlike applications built using other programming models. without wasting any further time. You just can't go ahead and build it right away. So. an object is built from an object design and object design is what we call as class. in OOP methodology. an object is a carbon copy of a class. An object is an instance of a Class An object is a runtime entity of a Class. we can build ‘n’ number of different cars from it. let’s see what it is before doing some programming using objects. Writing a program that creates objects from a class. So. Point to be noted. and defining the interactions between the objects is what we call as object oriented program and such a programming model is referred to as Object Oriented Programming model (OOP). and every car is an independent entity. So. OOP model is a very powerful programming model as it allows us to build reusable. So. “An object is an instance of Class”. a car is built from a car design. Cool. Likewise. so our definition of an object is. doors etc. we write object oriented programs and we write them in Java from here onwards. modular. we need a class. Let’s say you want to build a car. An object is a memory model of a Class. seats. is nothing but programming using Objects. to create an object. a computer is built using computer design. Once we have a car design.
Difference between a Class and an Object
In simple terms.. The central idea or entity on which OOP model is based is something called object.Chapter 4
Object Oriented Programming
What is an Object?
Introduction
Object Oriented Programming (OOP) as the name suggests.
55
. You first need to design a car. It has its own wheels. if we have a class. and then build the car. Similarly.
What is an Object?
There are several ways to define an object like the ones below. Another point to be noted. we can create ‘n’ number of different objects belonging to that class and again every object is an independent entity.

we learned something about class level methods and variables and object level methods and variables. No. Tough question. I’ll try my best to cut short the theory and explain things with examples.1 Relationship between classes and objects.Chapter 4
Object Oriented Programming
What is an Object?
Two important points to remember
1. we do the following three things. 2. Create objects of that class and 3. Define the interaction between the created objects.
Therefore. Now. For every class. Let‘s say we have a class with two variables and two methods. we can create ‘n’ number of objects belonging to the same class only. There is no ‘object’ without a ‘class’. Like you. 2. I do hate theory.1 shows the relationships between classes and objects
Class A
Class B
Class C
O1
O2
O3
O1
O2
O1
O2
Fig 3. If you recall from chapter 2. Yes. but you know what. if the variables and methods in the class are not declared as static. Let's stop this theory crap. to write an Object Oriented Java program. Fig 3. sometimes we can’t escape from it. Write a class like we did before. 1. the question is.
56
. does the object also have the same two variables and methods? Hmmm. But. let me tell you one thing. if the variables and methods in the class are declared as static. Now it’s time to clearly understand what this means. and see with an example to make our life easy. right? The answer is both Yes and No. if we create an object from this class.

then half the job is done. all a class will have is static and non-static methods and variables.methodname syntax. These are called object level properties.
package objectconcepts.Chapter 4
Object Oriented Programming
What is an Object?
Listing 4. when we create objects of this class.out.out. Objects will only get non-static variables and methods defined in a class So. and if you don't see static. static void display() { System.1 (Printer. I am sure you didn’t forget these. Trust me. which variables and methods the objects will have? To understand. here are two very simple rules. 2. mark them as object level fields. } }
The code shown in listing 4. the question is. } void print() { System.println("The value of i is " + i). int j = 20.println("The value of j is " + j). In Java. we’ll see how to access object level properties. mark them as class level. If someone gives you a class or if you need to use some library class. and to access variables we use ClassName. If you see the keyword static for a method or variable. From here onwards in this chapter.variablename syntax which we already discussed in chapter 2. This is the reason why we call class level properties (variables and methods) as shared properties. All the static methods and variables namely i and display() will remain in the class only.java) Class with object level and class level properties.
57
. 1. public class Printer { static int i = 10. every object created from this class will only have variable j and method print().1 has two variables namely i. If you could do this. we use ClassName. the first thing you need to do is identify which are class level fields and which are object level fields. Now. To access class level methods. These are class level fields and can be shared by all the objects created from the class.j and two methods namely display() and print().

1. it has two of them namely.java) Class with object level and class level properties. static void print() { if (paperCount > 0) { paperCount = paperCount . No confusions. System.1 (Printer.println("Please load the paper in the tray").Chapter 4
Object Oriented Programming
Objects and Methods
Note: For all the examples in this chapter. One is the class whose objects will be created. } }
As you can see from the code. two classes namely Car and CarTest.2. Objects created from this class will only have this method. the other is a test class that will create the objects and invoke the methods on these objects. we will write at least two classes. See listing 4. Listing 4. paperCount variable and print() method (static ones). } } void printDocument() { print(). we simply have to execute the test class. } else { System. public class Printer { static int paperCount = 1. right?
Objects and Methods
Object oriented programming is all about creating objects and invoking methods on objects. So.out. we will start creating car objects.out. To execute the program.println("Document printed"). let’s begin our journey into object oriented programming by writing a class that has both class level as well as object level properties. For instance. the Printer class has one object level method namely printDocument() method (non-static).
package objectconcepts. Coming to the class level properties. The test class is the one that will have the main method and it is in this method. These two properties can be shared by all the objects
58
.

The moment objects are created. it first created two printer objects. } }
As you can see from the above test class.printDocument(). }
Tip: For the class.
59
. Printer p2 = new Printer(). public class PrinterTest { public static void main(String args[]) { // Create 2 objects Printer p1 = new Printer(). Based on the above syntax. The syntax for creating an object from a class is shown below: <class name> <objectReference> = new <class name> (). In the above. p2.2 (PrinterTest.2. // Invoke the object level method p1.printDocument(). and references the objects using the names p1 and p2. Now. visualize a rectangle that has a print() method and paperCount variable. we created two printer objects as
Printer p1 = new Printer(). JVM will load the objects with all the object level properties (printDocument() method) .Chapter 4
Object Oriented Programming
Objects and Methods
created from this class by directly invoking them from the object level methods as shown below:
void printDocument (){ print(d).
package objectconcepts. look at the test class in listing 4. Both p1 and p2 are two completely different objects independent of each other.java) Test class to create printer objects. Printer p2 = new Printer(). new is the keyword used for creating objects. Listing 4.

we need to use referencename.
Document printed with 20 Please load the paper in the tray
Following picture demonstrates the working of this example. but class level methods cannot access object methods.Chapter 4
Object Oriented Programming
Objects and Methods
Tip: Visualize two circles (objects) pointed with the names p1 and p2 in your mind each having printDocument() method. Now you have one rectangle and two circles in your mind. it invokes the same shared print() method in the class. Now. which sees the paperCount as 0. This is the picture I asked you to imagine in your mind. Violating this rule causes compilation errors. Like an object level method invoking the class method.java C:/>JavaTraining>Chapter4>java objectConcepts.
The above statement invokes the printDocument() method in the object referenced by p1.printDocument().
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. See the following rule. decrements to 0 and displays the following message:
Document printed with 20
Similarly. This method in turn invokes the shared print() method in the class. Good job. let me ask you a question here. can a class method invoke object method? I must say big No to this. To invoke the object methods. Rule: Object level methods (non-static) can always access class level methods (static methods). and displays the following message
Please load the paper in the tray
This is how object level methods share the class level methods. The print() method checks the paperCount which is 1. when printDocument() method is called using p2 reference. Save the above two files in objectconcepts directory and compile them as shown below.
C:/>JavaTraining>Chapter4>javac objectConcepts\*.methodName syntax as shown below:
p1.PrinterTest
The result of the above program will be.

3a where the class only has object level properties. right there. imagine a circle(s) with the name of the reference variables or draw it on a piece of paper as shown in the above picture. Listing 4. We are strictly into object oriented programming business.Chapter 4
Object Oriented Programming
Objects and Methods
int paperCount void print()
void printDocument()
void printDocument()
p1
p2
Tip: The best and easiest way to understand objects is by visualizing them. See listing 4. // Instance methods public void sum(int i.java) Class with only object level properties. } public void multiply(int i. Every time we create object(s) using new keyword. we are going to write classes with only object level methods and variables.3a (Calculator. int k) { result = i * k. from here onwards in this chapter. } }
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. int k) { result = i + k.
package objectconcepts. Ok. Do this for the first few examples. public class Calculator { // Instance variable double result = 0. Now that you know how to create objects from class and invoke methods using their references.

16).out.sum(4. This means. invoke both the methods on the same object as shown below:
calc1. it creates another object cal2 and invokes the multiply() method and updates its own result variable. cal2. all object level methods can share all the object level variables.multiply(5.result).result).result). and update the same result variable. The test class then displays the value in the result variable using the following statement. we are invoking only the sum() method in one object.result). public class CalculatorTest { public static void main(String args[]) { // First object Calculator cal1 = new Calculator(). cal1.sum(4.10). You can however. System.out.println(cal1. calc1. System.println( cal1.
The above statement updates its result variable to 20. and multiply() method in the other. Listing 4. Both the methods take some data.
Similarly. Though the object has both the methods sum() and multiply().3a has two instance methods namely sum() and multiply() and one instance variable result.Chapter 4
Object Oriented Programming
Objects and Methods
The calculator class in listing 4.5).multiply(5. System. // Second object Calculator cal2 = new Calculator().sum(10.out.out. System.println(cal2. } }
As you can see from the above test class.java) Test class creating multiple objects. it created two calculator objects and invoked methods using their respective references as shown below:
cal1.println( cal1.result). 6).println( cal1. 16).3b (CalculatorTest.
Compile both the classes and execute the test class like as shown below
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.out.
package objectconcepts.
System.

we created two independents as shown in the above figure and just invoked the methods on these independent objects. Once you are done scratching the carbon copy.2 should give some idea. so let’s learn how to tie these objects together and make them more powerful. you can take another copy (create another object) and start working with it in a different way (invoking different methods on the object). As I said right at the beginning of the chapter.java C:/>JavaTraining>Chapter4>java objectConcepts.Chapter 4
Object Oriented Programming
Objects and Methods
C:/>JavaTraining>Chapter4>javac objectConcepts\*. One thing we should understand is having independent objects simply serves no purpose. Fig 4. we need to have both objects interact with
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. then to send the message successfully to the legacy system. If we have two objects with one operation in each.
calc1
calc2
double result = 20 void sum() void mutiply()
double result = 30 void sum() void mutiply()
Fig 4. A picture is worth 100 words.2 Objects associated with respective references
In the above example.CalculatorTest
The output of the above program is
20 20
Think of objects as carbon copies made from the original document. an object oriented program creates objects as well as establishes communication between these objects. We see two operations here namely ‘sending message’ and ‘formatting message’. We now know how to create objects. You can play with every carbon copy (invoking methods on object) as you like. Let’s say we want to send a message to a legacy system which requires the message to be formatted and then sent.

System. the test object invokes the sendMessage() method on the Messenger object. }
Listing 4.out. Listing 4.4 shows the code for the above messenger and formatter classes. the former must first create the object of later.3 Object communication
The communication between the above two objects will be established by having the sendMessage() method of one object invoking the formatMessage() method of the other .Chapter 4
Object Oriented Programming
Objects and Methods
each other. for one object to invoke the method on the other object. which in turn invokes the formatMessage() method on the Formatter object. public class Messenger { public void sendMessage() { // Invoke the method in the formatter object Formatter f = new Formatter().3.
package objectconcepts. } }
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. and then invoke the method as shown below:
void sendMessage(){ Formatter f = new Formatter().println("Message sent successfully to legacy system"). System. In Java. In this case. f. f. formatMessage(). as shown in Fig 4.formatMessage().
Messenger
Formatter
Test class
void sendMessage()
void formatMessage()
Fig 4.4a (Messenger.println("Message sent to legacy system").out.java) Class to send message.

Finally.java C:/>JavaTraining>Chapter4>java objectConcepts.4c (MessengerTest.4b.java) Test class to send a message. Listing 4. } }
If you look at the messenger class in listing 4.sendMessage().
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.println(" Message formatted").
C:/>JavaTraining>Chapter4>javac objectConcepts\*.
package objectconcepts. public class Formatter { public void formatMessage() { System. This is how messenger object interacts with formatter object. The rule is pretty simple. it should simply create object(s) and invoke the methods. Before we look at the next concept.4a. public class MessengerTest{ public static void main(String args[]){ // Create a messenger object Messenger m = new Messenger().out.4c.Chapter 4
Object Oriented Programming
Objects and Methods
Listing 4. right? The Formatter class is pretty straightforward as it simply defines the formatMessage() method as shown in listing 4.
Message formatted Message sent successfully to legacy system
This is how objects interact with each other.4. its sendMessage() method created an object of Formatter class and invoked the formatMessage() method. the test class simply creates an object of messenger class and invokes the sendMessage() method as shown in listing 4. } }
Compile and execute all the three classes as shown below.
package objectconcepts.MessengerTest
The result of the above test class will be.4b (Formatter. If an object should interact with any another object(s). simple. let’s do one complex example where several objects interact with several objects as shown in Fig 4.java) Class to format the message. // Send the message m.

} } package objectConcepts.println ("I am in } } package objectConcepts. Foo object interacts with Helper2 and Helper4 objects.Chapter 4
Object Oriented Programming
Objects and Methods
Foo m1()
Helper2 m2()
Helper3 m3()
Helper4 m4()
Helper5 m5()
Fig 4. there could be several objects interacting with each other in much more complex fashion. Also. Foo object is the one that initiates the interaction. However in real world applications. and you’ll get the complete picture.5a for each of the above classes.4
package objectConcepts. Listing 4.out.4 Complex interaction of objects
From the above figure.println ("I am in Helper3"). and Helper2 object interacts with Helper3 and Helper5 objects.
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. public class Helper3 public void m3(){ System. public class Helper4{ public void m4(){ System.println ("I am in Helper5").out.out.5a Code snippets for the classes shown in Fig 4. Look at the code listing 4. } } Helper4"). public class Helper5{ public void m5(){ System. Each class has methods as shown in the figure.

The result of the above test lass will be is
In Foo I am in I am in I am in I am in Helper2 Helper3 Helper5 Helper4

Understanding Object References
So far we have seen quite a few examples that created objects using new keyword, and assigning them to object references which we then used to invoke the object methods. Though this looks pretty simple and straightforward, we need to know how the JVM internally uses the object references to invoke the methods. Let’s spend few minutes here and understand the nuances of object references. Look at the following class.
public class Messenger { String msg = ""; public void sendMessage() { System.out.println("Message" + msg + "Sent"); } public void setMessage(String message) { msg = message; } }

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The above Messenger class has two methods. One method that sets the message and the other method that sends the message. With this class, let’s create an object as shown below:
Messenger m1 = new Messenger();

The above statement creates the following representation in JVM’s memory.
m1

sendMessage() setMessage()

The reference variable ‘m1’ is like a pointer to the physical object. It is using this pointer, we invoke the methods in the object as shown below:
m1.setMessage("Hello "); m1.sendMessage();

Now, let me ask you a question. Can an object have more than one reference pointing to it? Yes it can. This is achieved by assigning references. Assigning object references is very much like assigning primitive variables. Look at the following two statements.
Message m1 = new Message(); Message m2 = m1;

As you can see from the above statements, we first created an object with reference ‘m1’. We then assigned m1 reference to m2 reference just like we assign primitive variables. The only rule is that the reference variable m2 should also be of the same class type. Now the object in memory will have two references as shown in the following figure.

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m1

m2

sendMessage() setMessage()

We can now use any of the above references to invoke the methods as they are pointing to the same object. For instance, we can use ‘m1’ to invoke one method, and ‘m2’ to invoke the other method as shown below:
m1.setMessage("Hello"); m2.sendMessage();

In Java, a single object can have ‘n’ number of references, and we can use any of the references to update the objects properties. Now that we know how the object references work, the question that comes to our mind is, when and why do we assign references? Good question. The main application of assigning references is when passing ‘objects’ as parameters to methods. In Java, just like we pass primitive data types (int, float etc) as arguments to methods, we can also pass objects as parameters. Look at the code in listing 4.6 that demonstrates assigning references. Listing 4.6a (Book.java) Simple class with methods and variables.
package objectconcepts; public class Book { double bookPrice; public void setPrice(double price) { bookPrice = price; } public double getPrice() { return bookPrice; } }

The Book class shown above is pretty straight forward. It defined two methods, one to set the price, and the other to return the price.

The above test class defined a method displayPrice() that echoes the book price. To see how this works, the main method first created the book object ‘b’ and set the price by invoking the setPrice() method. At this point, the book object will be as shown below:
b

bookPrice = 24.99 setPrice() getPrice()

The main method then passed the reference ‘b’ as a parameter to displayPrice() method. Since ‘b’ is of type Book, the method parameter in the displayPrice() must also be of type Book as shown below:
static void displayPrice(Book b1)

When ‘b’ is passed into ‘b1’, it is indirectly equal to the following assignment:

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b1 = b;

Therefore, ‘b1’ also points to the same object as ‘b’ points and the book object will now have two references as shown in the following figure.
b b1

bookPrice = 24.99 setPrice() getPrice()

Since ‘b1’ is pointing to the same object, it is like we indirectly passed the object to the method. But, strictly speaking we passed the object reference, but not the physical object, right? So, when the method invokes getPrice() method using the reference ‘b1’ as shown below, it returns the price of the book object.
double price = b1.getPrice();

Therefore the output of the above program will be,
24.99

This is how we pass objects to methods. Passing objects as parameters to method is another way of making the objects interact with each other. Now that we know how to define interactions among different objects, it’s time to know something about two important keywords public and private when used with object methods and variables. These are also known as “access modifiers”. We all know without any hesitation that an object has methods and variables and we all know that objects interact with other is by calling each others methods. Let’s say I am an object for now. I have some personal information that I don’t want to share with other objects, but have some other information that I really want to share. So, how can I restrict other objects from accessing my personal stuff and only access what I wanted them to? Is there a way? Yes, there is one. Simply prefix those methods and variables you think are personal with private keyword, and for the ones you want to share, prefix with public. That’s it. If some other class or object tries to invoke private methods, the

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Encapsulation

compiler spits an error message. Based on these two keywords, let’s look at the first OOPS concept, Encapsulation.

Look at the TaxCalculator class. It has three private variables and three private methods and no class can directly access these properties. If they try, you know what happens. Compilation errors. It has one public method getMyReturns(). This is the only method any one can ever access. The getMyReturns() method internally uses the private methods as helper methods to computes the taxes. The method finally returns the results back. This method is hiding the details and giving only the essentials. Isn’t this what encapsulation is all about? Cool.

Now, look at the TaxCalculatorTest class. This class creates the tax calculator object first, and sincerely calls the public method getMyReturns()by passing all the required data (essentials). It never even dreams to call the private methods as it knows what happens. You can try calling the private method if you want. If you noticed TaxCalculator class carefully, the use of private and public keywords makes complete sense, right? As a user of the tax calculator, I really don’t care how it computes my taxes (details). All I care is how much tax returns it gives me back (essentials), right? This is what we call as give and take system. Compile and execute the programs to see the following result.
My total tax returns = 1190.0

Thumb Rule: Keep all variables private, and all methods either private or public. Never expose the object state (variables) to outside world. If some one needs the data in the variable, write a method that returns it. Ok. At any cost, make sure that an external

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object can only interact through methods. No direct access to variables should be allowed. This is a good practice. You know what; I am tired of using int’s and double’s in my programs for declaring the data. Storing text data is the common thing that we would see in any application since most of the data is text based. For instance, things like name, address, city, country and what not. So, I need a new primitive data type to store just text data. Do we have one? The answer is very big NO. Shoot! What should we do now? You know what, don’t worry. There is a special entity called as String which is not really a primitive data type, but behaves in a similar way. We will start using this from now onwards to store texts, but don’t even worry about its details for now. Just use it like any other data type. Ok. We’ll see its details later. There is a big section dedicated to it. Trust me. The usage of String is shown below. The text to be stored must be enclosed in double quotes.
String s = "Thank God, finally I found one for storing text data ";

Let’s get back to encapsulation for a moment. In real world applications how and where do we use it? Let’s say we want to store the details of a customer like firstName, lastName, address, ssn etc,. A good solution is to create a class called Customer, and define variables like firstName, lastName etc,. To, get a clear picture, see the code in listing 4.8a. Listing 4.8a (Customer.java) Customer class using encapsulation.
package objectconcepts; public class Customer { private String firstName; private String lastName; private String address; public void setFirstName(String fname) { firstName = fname; } public void setLastName(String lname) { lastName = lname; } public void setAddress(String add) { address = add; }

Damn, the programs are getting longer. You know what; the programs are instead getting simpler and classier as we move forward. Length doesn’t matter for us. Concept matters most. Look at the Customer class and see how simple it is. All it did is, 1. Declared three private string variables for storing text data. 2. Since another class cannot directly access these private variables, to facilitate storing of data, it defined three set methods to accept the data which will then simply store in the private instance variables. 3. To again return the data back, it defined three get methods to return the data in the private instance variables. This class simply encapsulated the variables and provided getter methods and setter methods for storing and retrieving the data. We call such a class as JavaBean or a POJO (Plain Old Java Object). In real world applications, to store complex data, there will be several of these classes. They simply declare private variables and define getters and setters. Don’t be surprised even if you see thousands of above like classes. Note: Some people call the above class as JavaBean and some others call it as POJO. Technically, both are same,ok. Now, look at the test class shown in listing 4.8b. Listing 4.8b (CustomerTest.java) Test class using the customer bean.
package objectconcepts; public class CustomerTest { public static void main(String args[]) { // Create the Customer Object Customer c1 = new Customer();

As you can see from the above test class, it created Customer object c1 and invoked the set methods to store the data, and then invoked the get methods to retrieve the data back. Compile and execute the test class to see the following result.
John Smith Grove City, CA

In the above customer class, I see one small bad practice, the names of method parameters in the set methods. For instance, look at the following set method:
public void setFirstName(String fname){ firstName = fname; }

In the above method, the argument name is fname, and the instance variable name is firstName. We are using two different names to represent the same data, right? In such situations, it’s a good practice to use the same names for both. If this is the case, then which one refers to instance variable and which refers to argument variable? To solve this dilemma, we need to use the keyword this as shown below.
public void setFirstName(String firstName){ this.firstName = firstName; }

In the above method, this.firstName refers to instance variable, and firstName refers to argument variable name. Likewise, to be consistent, in the get methods we should return the instance variables as

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Method Overloading

public String getFirstName(){ return this.firstName; }

The above technique of using this keyword is called as instance variable hiding. Modify all the methods in the Customer class using the above convention, and you’ll see the same results. The above case of encapsulation is very simple, yet very powerful. It offers restricted access to methods and variables by using private and public keywords. Trick: Always try to visualize the objects as circles and method names within these circles for the first few examples. This makes understanding objects easier. Let me once again reiterate that to work with objects, all we need to do is 1. Create an object using new keyword and 2. Start invoking the methods using the object reference. Now, let’s learn another concept called method Overloading, another classical OOPS concept.

Method Overloading
Method Overloading is a mechanism where more than one method in a single class shares the same name. Following are the two rules for overloading methods. 1. One or more methods in a class must have the same name 2. The number of arguments or the type of arguments must be different. If the above two rules are satisfied, then we say that the method is overloaded. Method overloading is very useful in some situations where you have one generic operation that can be implemented in number of ways. For instance, let’s say we have a messenger whose sole purpose is to send text messages to a mainframe system. Now, what if this messenger should have three sending operations to send one text message, two text messages and three text messages based on the situation? This is like same generic operation but different behavior. Such scenarios are best implemented using overloading. Listing 4.9a demonstrates method overloading.

If you look at the MultipleMessenger class, we overloaded the sendMessage() method. This method obeys both the rules of overloading (same name but different number of arguments). This feature allows us to use the same names for methods whose high-level behavior is the same (send messages in our case). Now look at the test class shown in listing 4.9b. Listing 4.9b (MultipleMessengerTest.java) Test class invoking overloaded methods.
package objectconcepts; public class MultipleMessengerTest { public static void main(String args[]) { MultipleMessenger messenger = new MultipleMessenger(); // Invoke the overloaded methods messenger.sendMessage("Hello "); messenger.sendMessage("Hello", "World"); messenger.sendMessage("Hello", "Beautiful", "World"); } }

As you can see from the above code, we invoked all the three methods passing different data. Based on the data, JVM will figure it out as to which overloaded method in the messenger class it should invoke. Isn’t this cool? I am sure you are with me.

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Another point to be noted is that you can also have another set of overloaded methods in the same messenger class. For instance, you can write three more formatMessage() overloaded methods next to the sendMessage() methods in the same messenger class. There is absolutely no limitation on the number of methods you can overload, as long they adhere to the two rules of overloading. You’ll see a lot of method overloading in real world applications. As an example, we can get a customers profile based on last name, or based on first name and last name, or based on SSN etc. In all the cases, the number of parameters or the type of parameters is changing, but the operation is still the same, right? In such cases, it’s a good practice to use method overloading. Compile the above two classes and execute the test class to see the following result.
Hello sent to mainframe system Hello World sent to mainframe system Hello Beautiful World sent to mainframe system

Now, here is a certification question you can expect. Look at the following two methods and tell me whether they are overloaded or not?
public int sum( int n1, long n2); public int sum( long n1, int n2);

You are correct. They are overloaded. If you look at rule 2 of method overloading, it says Number of arguments or Type of arguments must be different. In this case, the type of arguments is different. So, they are legally overloaded. Good. Since they are legally overloaded, let’s say my test class passes two numbers to the overloaded method as shown below:
c.sum(10, 10);

Now you tell me, which of the above two methods will be invoked? Most probably you are stumped. You know what, JVM is also stumped, and gives a compilation error saying “method sum is ambiguous”. Here is how the compiler interprets the code. It treats both 10 and 10 as int and looks for a method that takes int and int. When it cannot find such method, it looks for a method whose parameter combinations are equal to or greater than int. To its surprise, it finds two methods and gets confused as to which method to invoke. Any time the compiler gets confused, its only weapon to show frustration is by echoing an error. So, the solution is, either remove one of the above method, or pass the arguments correctly as shown below:

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c.sum( 10, (long)10) // to invoke the first method c.sum( (long)10, 10) // to invoke the second method.

The above statements use explicit data casting to invoke the right method. This is just to demonstrate the highly improbable scenario, but it’s always good to know such things. Things like this are frequently asked in certification exams. If they confuse you, simply ignore them. You’ll not see them in real world applications. But understanding these things will surely boost your confidence by many times. Just as a recap, at this juncture, we learned the following things 1. 2. 3. 4. How to create an object using new keyword How to invoke methods in objects using reference variables How to implement encapsulation using private and public keywords and How to overload methods in a class based on two simple rules.

Now, we will look at one of the most important feature in object oriented programming, “Object Initialization”. If you noticed carefully, in all the examples thus far in this chapter, we simply created objects and invoked its methods. We never initialized objects. Did we? It’s time now to start initializing objects. Let’s rock.

Object Initialization
If you recall from chapter 2, we learned something about class level initialization using the so called static blocks. Let me ask you a question. Since objects are created from a class, if we initialize a class, does this mean we also initialized all the objects of that class? I have to say ‘no’ again. An object though created from a class, can still have its own initialization. Let’s consider a classical printer example that prints documents in both color as well black and white. Printer can be viewed as a class, and documents as objects of printer, right? We need to feed the printer with papers. This is printer (class) initialization. Let’s say the printer must print documents in both black and white as well as color. Here color is specific to the document (object), and not to the printer. Choosing the color of the document (object) is object level initialization. Make sense? So, the question is, how do we initialize objects? We initialize objects using something called constructor. Let’s see what this is.

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Constructor
Constructor
A constructor is a special method defined in the class used for object initialization. Can a constructor take arguments or parameters like a method? Absolutely. not to take any arguments. its name is same as class name and doesn’t have any return type like void or int preceding the method name adhering to the constructor rules. this is all automatic. Special? In what way is it special? Look at the following. Let me ask you a question here. Sweet! So.
Printer p = new Printer(). Note this point. A constructor is a method in a class whose. Now we should agree that it’s indeed special. the constructor of this class would be as shown below:
Printer() { }
As you can see from the above constructor definition. right? Now that it identified the constructor. When we create an object of this class as shown below. Let’s look at the details as to how JVM invokes the constructor. Did we? So. we can also have constructors that take arguments. JVM invokes the constructor. Next thing to know is. and invokes it immediately after creating an object using new keyword. Consider a class named Printer.
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. who invokes the constructor and when is it invoked? Great question. why not? We didn’t specify anything in the constructor rules. Name must be same as class name and 2. All we need to do is define a constructor in the class with what ever initialization we want to do for the object and leave the rest to JVM. Must not have a return type. right? We don’t have to worry about invoking constructors at all. So. Simple. It is using these arguments that we do different initializations for different objects.
It is at this point. it simply executes the statements inside the constructor method right away. based on the above 2 rules. JVM identifies the constructor based on the characters after new keyword highlighted in bold. 1.

Test. I think it’s time to write an example using constructors. Inside the constructor. JVM will invoke the constructor that takes int argument and assigns the number to the constructor argument val. So. the object uses the val variable for its initialization. let’s say we want to pass an integer value to a constructor of some class say. } public void printDocument(String text) { System. Let’s implement the printer and documents idea we discussed in the beginning. } }
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.10a (AdvancedPrinter.out.
Note: If you define a constructor and don’t pass the arguments correctly. Listing 4.java) Class using constructor. To create another object and initialize with say 20. public class AdvancedPrinter { String printType.
Test (int val){ // Some initialization statements here }
To create an object that uses the above constructor.println(text + "printed in " + printType).Chapter 4
Object Oriented Programming
Constructor
As an example. Listing 4. we need to pass the integer number right at the point of creating the object as shown below:
Test t1 = new Test(10). The constructor of this class would then look as. you get compilation errors.
Since 10 by default is of type int.
package objectconcepts. // Document initialization AdvancedPrinter(String type) { printType = type. we simply do it as shown below:
Test t2 = new Test(20).10a shows the printer class. be careful.

Listing 4.
package objectconcepts. we learned that JVM invokes the constructor at the time of creating an object. AdvancedPrinter colorPrinter = new AdvancedPrinter("Color").
Yet the examples compiled and executed fine. here is the tricky situation. Didn’t they? However.Chapter 4
Object Oriented Programming
Constructor
As you can see from the printer class in listing 4. When the printDocument() method is invoked. Now look at the test class shown in listing 4.printDocument(" Test Document 1"). So.10a. it defined a constructor that takes type as an argument and initializes the instance variable printType. So. we passed ‘Black And White’ as the print type. how is this possible when none is defined? Stumped again? Don’t
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. we didn’t define any constructor and we created objects as shown below:
Display d = new Display(). bw. we get the following displayed.10b (AdvancedPrinterTest. Compile and execute the test class to see the results. during the discussion on constructors. when we invoke the printDocument() method passing the text. Now that you know how to define a constructor. and displays the following when the printDocument() method is invoked. right? We were able to create objects with different initializations. In all the several examples we wrote thus far. public class AdvancedPrinterTest { public static void main(String args[]) { AdvancedPrinter bw = new AdvancedPrinter("Black And White").10b.java) Test class invoking the constructors.
Test Document 1 printed in Black And White
The colorPrinter object passes ‘Color’ as the print type.
Test Document 2 printed in Color
Simple and cool. For the first object. and how to create objects to invoke the constructors. it uses the printType to print the text. colorPrinter.printDocument(" Test Document 2"). } }
The above test class simply created two printer objects ‘bw’ and ‘colorPrinter’.

public class Computer { int ramSize. int hardDiskSize. A default constructor is nothing but an empty constructor with no parameters and no body as shown below:
Display() { }
Though we didn’t define any constructor in our examples.11a (Computer. can we also overload constructors? You’re right. This is the reason why all the examples compiled and executed fine without any problems. constructor is a special method. Like we overloaded methods. Listing 4. JVM provided us with an invisible to us and visible to JVM default constructor.Chapter 4
Object Oriented Programming
Constructor Overloading
worry. String os. This means an object can have different initializations or more specifically different levels of initialization. visualize the presence of a default constructor. Here is the rule. public Computer() { // Default Initialization ramSize = 512. See. os = "Windows XP Home". let me ask you a simple question. Take a look at listing 4. }
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. :-)
Constructor Overloading
As I said before. We can well and truly overload constructors. Moreover. } public Computer(int rs) { // Initialization with differnet RAM size ramSize = rs. JVM is so generous. the same rules apply for constructor overloading. os = "Windows XP Home". hardDiskSize = 40.11a. If no constructor is defined. When a class doesn’t define any constructor. JVM is gracious enough of provide a class with a “default constructor”. hardDiskSize = 40. If this the case.java) Class with multiple constructors
package objectconcepts.

os = "Windows XP Home". hardDiskSize = hd. Listing 4. This is the most specific constructor. The compute() method simply uses the values initialized by the constructors for computing.Chapter 4
Object Oriented Programming
Constructor Overloading
public Computer(int rs. int hd) { // Initialization for both RAM and HD size ramSize = rs. int hd.out. Similarly. A computer object created using this constructor is always a Windows XP Home with specified hard disk and RAM capacity. A computer object created using this constructor is always a Windows XP Home with 256 MB RAM and 40 GB hard disk. public class ComputerTest {
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. String os1) { // Initialization for all the three ramSize = rs.java) Test class using different constructors for creating objects. } }
The above code defines a Computer class with four overloaded constructors. The third constructor allows to create objects with different RAM and hard disk size but with default OS.11b.11b (ComputerTest. The second constructor allows the objects to specify the RAM capacity with other two variables to default values.println("Computing with " + os + " with " + ramSize + "MB RAM and " + hardDiskSize + "GB hard disk"). } public Computer(int rs. os = os1.
package objectconcepts. } public void compute() { System. Now look at the test class shown in listing 4. A computer object created using this constructor is always a Windows XP Home with 40 GB hard disk and a specified RAM capacity. One is default constructor that initializes all the instance variables to default values. the last constructor allows us to specify all three parameters. hardDiskSize = hd.

right? There is one important rule with invoking the constructors. it needs to use this keyword with the constructor arguments. is there a way by which we can eliminate these duplicate initialization statements? Yes. 80. Computer(512.
Computing Computing Computing Computing with with with with Windows XP Home with 512MB Windows XP Home with 256MB Windows XP Home with 512MB Max OS with 1024MB RAM and RAM and 40GB hard disk RAM and 40GB hard disk RAM and 40GB hard disk 80GB hard disk
If you noticed carefully at the Computer class. the corresponding constructor will be invoked. For instance.compute(). Otherwise. to call the default constructor we need use the following statement:
this(). we use the following
this(10).compute(). 40). c2. Computer(256). } }
The above test class creates four different types of computer objects using different constructors. The above statements must appear as the first statement inside the constructor. So. "Max OS"). c3 the third and c4 the fourth constructor. a compilation error
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.
Similarly.
os = "Windows XP Home ".
The above statement appears thrice and some other statements too.compute(). c4. Based on the parameters passed.Chapter 4
Object Oriented Programming
Constructor Overloading
public static void main(String args[]) { Computer Computer Computer Computer c1 c2 c3 c4 = = = = new new new new Computer(). For one constructor to invoke another constructor.
c1. right? Compile and execute the test class to see the following result. No complexities and confusions. c3. c1 uses default constructor. Computer(1024.
The above representation is like replacing the constructor name with the this keyword. to call the constructor that takes an integer argument. you’ll see some duplicate initialization statements such as.compute(). c2 invokes second constructor. We can solve this problem by having constructors invoke each other.

if we get another client needing for 6 methods. we need at least two classes.Chapter 4
Object Oriented Programming
Inheritance
This is how we overload constructors and have them invoke each other. we will come across constructor overloading for sure. right? At this point. copy all the four methods from the old class and add the one new method. compile and execute the test class to see the same results.12a. Tomorrow. we again have to update the class. and therefore we are impacting client 1 as well as client 2. One is the parent class whose properties will be inherited. have the new class automatically inherit the functionality that it needs from the old class so that it doesn’t have to duplicate the code and just define the new functionality. let’s define what inheritance is. Aren’t we? This is a very bad practice. but why do you want to give additional functionality to client 1 when he doesn’t need it? Moreover. Properties are nothing but variables and methods.
Inheritance
First. called as the child class. Let’s say we wrote a class today with four methods for client 1. Listing 4. As we write many more examples throughout this book.java) Calculator class for client 1
package objectconcepts. a child class cannot inherit the private members of the parent class. We now arrived at a point where we are ready to learn the most important OOPS concept. public class BasicCalculator {
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. As always. it’s important to understand as to when we use inheritance. look at the code in listing 4. what we need to do is simply create a new class. and the other is the class that inherits the properties. the Inheritance. This is exactly what inheritance allows us to do. you might wonder why not just add the one new method in the old class? You can do. If we don’t have inheritance. during inheritance. For applying inheritance. Before we write programs. we got a new client and needs the same four methods plus one additional method. The keyword extends is used to create a child class from a parent class. Inheritance is a mechanism where one class acquires or inherits the properties of another class. The best solution is. Moreover. To better understand inheritance.12a (BasicCalculator.

Listing 4. Now look at the test class shown in listing 4. Listing 4. This class then defined the new multiply() method which leads to this class having both sum()that is inherited as well as the new multiply() methods. double m) { result = n * m.12c that verifies the inheritance. BasicCalulator is the parent class and ScientificCalculator is the child class. double d2) { result = d1 + d2. return result. Though you don’t physically see them.
package objectconcepts. return result.java) Calculator class for client 2. we created a ScientificCalculator class for client 2 that extends from the BasicCalculator class as shown below:
public class ScientificCalculator extends BasicCalculator
The above statement will tell the JVM to inherit (copy) all the non-private methods from the BasicCalculator to the ScientificCalculator. they are implicitly there in the class. See listing 4. Now we got a new client 2 who needs the sum() method as well as multiply() method. You have to visualize their presence. } }
Assume that the above BasicCalculator class is sold to client 1.Chapter 4
Object Oriented Programming
Inheritance
private double result. Notice that the private variable in the BasicCalculator class will never be inherited which is why the child class declared its own result variable. public class ScientificCalculator extends BasicCalculator { private double result. public double sum(double d1.
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.12c (CalculatorInheritanceTest.java) Test class for client 1 and client 2
package objectconcepts. public double multiply(double n.12b. } }
As you can see from the above code. Here.12b (ScientificCalculator.

We can then create an object of this class and invoke all the three methods as shown below. multiply() and divide() ? Do you want to. Isn’t this cool and classical?
AdvancedCalculator ac = new AdvancedCalculator(). 30). B is the parent of C as shown in the following figure.
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. This is client 1 code. This is exactly what we should and can do.multiply(20.10).
The above example follows the pattern in which A is the parent of B. Next.sum(20. result = sc.sum(10.10).
public class AdvancedCalculator extends ScientificCalculator
The above class inherits the sum() as well as multiply() methods and defines the divide() method. AdvancedCalculator will now be the child of ScientificCalculator and following is how its class declaration will be. ac. result = sc. } }
As you can see from the above test class. ac.Chapter 4
Object Oriented Programming
Inheritance
public class CalculatorInheritanceTest { public static void main(String args[]) { // Client 1 code BasicCalculator bc = new BasicCalculator(). 10). // Client 2 code ScientificCalculator sc = new ScientificCalculator(). who needs the methods sum(). double result = bc. 20). ac. This is possible only because the child class inherited the sum() method.sum(10. 30). What if I got a new client. Isn’t this cool? Now a small question.multiply( 10. it first created an object of BasicCalculator class and invoked the sum() method.divide(10. a) Write a class AdvancedCalculator that inherits from BasicCalculator and define multiply() and divide() methods (or) b) Write a class AdvancedCalculator that inherits from ScientificCalculator and just define the divide() method I am sure you would go with choice b. it instantiated the ScientificCalculator class and invoked both the sum() as well as multiply() methods.

However. For instance. we learned how to create a child class from a parent class.Chapter 4
Object Oriented Programming
Inheritance
A
B
C
In the previous example. I cannot write the entire method in the figure.m3 : void
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. Look at the following block that represents a class in our diagram. Since. let’s use some conventions.val : String + m1 : void + m2 : void . Let me ask you another question here. There is absolutely no limit on the number of child classes a parent class can have.
Class: A + j : int . Can a parent class have just one child class? No. To demonstrate the real power of inheritance. we can also create another child from it like MathCalculator as shown below:
public class MathCalculator extends BasicCalculator { }
This child class will now have sum() method plus its own methods. A parent class can have any number of child classes. let’s look at a complex inheritance hierarchy diagram and understand what methods are inherited by various classes in the diagram. in the above example BasicCalculator has just one child called ScientificCalculator.

+ represents public members.5 that represents a complex inheritance hierarchy. and methods m1.m4.m5. let’s see that methods and variables various classes will have. we observe the following things. D. B. Class B: From A it inherits Variables: j Methods: m1 and m2 Combining the above with its own variables and methods. E. Now.B. and – represents private members. Based on the above rule.Chapter 4
Object Oriented Programming
Inheritance
In the above class block. F each having their own methods and variables.m6.E is another branch A. the above block represents a class named A having one public variable j. two public methods m1 and m2 and one private method m3. one private variable val. A.D is one branch A.B. B will have j and val as variables. Based on this convention. From the class diagram. The inheritance diagram shows six classes namely A. C.m2.F is another branch A is the ultimate parent class B and C inherit from A D inherit from both B and A as they belong to the same branch E inherit from both B and A as they belong to the same branch F inherit from C and A as they belong to the same branch Note: Private variables and methods are never inherited.C. (See the figure in the next page)
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. look at the Fig 4.

val as variables. m5 Combining the above with its own variables and methods. m4. m2. k. val as variables. m14.Chapter 4
Object Oriented Programming
Inheritance
Class D: Inherits from both A and B From A. Now. m17. m2. m13. m5. and methods m1. C it inherits Variables: j. B it inherits Variables: j Methods: m1. let me ask you couple questions here. m18. m5 from existing classes and a new method p1. I hope the above picture and the derivations are pretty straightforward. Class E: Inherits from both A and B From A.k Methods: m1. m10. m8. m4. and methods m1. B it inherits: Variables: j Methods: m1. This is how we can build the inheritance hierarchy and determine which methods and variables are inherited. E will have j. and methods m1. m2. m12. val as variables. Class F: Inherits from both A and C From A. m16. m2. m8 Combining the above with its own variables and methods. m15. m4. m4. D will have j. m7. m2. m7. m2. m5 Combining the above with its own variables and methods. D will have j. m4. m2. m11. m5. Question 1: I need to build a class with the methods m1. Which of the following classes my new class must extend:
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. based on the above diagram.

only single inheritance is allowed. we don’t see a class having more than one parent. which is also known as multiple inheritance? The answer is big no. multiple inheritance is strictly not allowed which means we cannot create a child class from two different parents. The reason why Java doesn’t allow multiple inheritance is that it introduces whole lot of confusions and complications which is fundamentally against the primary goal of Java. I am sure you got them right. If you carefully observe the above inheritance diagram. Therefore the following class declaration causes compilation error. m8. in Java. So the question is.6 shows both legal and illegal inheritance hierarchies. Which of the following classes my new class must extend: a) Class F b) Class C The correct answers are ‘choice b’ for Q1 and ‘choice a’ for Q2.
A
B
A
C Illegal
A
A Legal
Fig 4. can a class have more then one parent. Fig 4. “Keeping things Simple”. m16. m7. m2. m17 from existing classes and a new method p2.6 Legal and Illegal Inheritance
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.
public class G extends A.B // Not allowed in Java
All this is to say that. Now you are an expert in inheritance and its intricacies. A class can only have one parent.Chapter 4
Object Oriented Programming
Inheritance
a) Class A b) Class B Question 2: I need to build a class with the methods m1. In Java.

Now. Method overwriting requires us to overwrite the inherited old method with the new updated method.
Method Overwriting
We all now know that we use inheritance when we need to create a class with most of existing functionality plus some new functionality. More and deeper details on inheritance are investigated in the next chapter.out. Good.13. right? So.println("Message Formatted"). how do we want to create this new class? Shall we copy all the methods from the old class and update the method that needs to be updated or use inheritance to eliminate code duplication? So. public class MainframeMessenger { public void formatMessage(String str) { System.13a (MainframeMessenger. Good.Chapter 4
Object Oriented Programming
Method Overwriting
This completes the basics of inheritance.java) Class for client 1
package objectconcepts. by the rule of inheritance the new class inherits the exact same methods that are in the parent class. What shall we do? Shall we update the method in existing class? No. let’s say we have a class named MainframeMessenger with three methods being used by client 1. Looks like we are back to square one. To clearly understand. The only solution for the above scenario is to create a completely new class. Listing 4. Now let’s say we got a new client who needs a new messenger class with same functionality as the above class. right? This is where method overwriting comes into picture. Hmmm. ok). what happened is. since our new SAPMessenger class is exactly same as MainframeMessenger class at this point of time. For now. I therefore created a new messenger class from the mainframe messenger class as shown below:
public class SAPMessenger extends MainframeMessenger { }
When we created our new class as above. but with slight modification to one of its methods (no new method this time. }
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. we can’t. let’s move on to the next concept. at this point the best solution is using inheritance. this is clearly not the solution. The challenge we have now is to update the inherited method. take a look at the code in listing 4. Again. because the updates made to method for client 2 will surely break client 1 functionality.

and overwriting just one method eliminates lot of code duplication. Now look at the code in listing 4. and then changing the behavior is what we call as method overwriting. In the above code.java) Class for client 2
package objectconcepts.13b (SAPMessenger. This is how it works.
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. This class will now have the old formatMessage() method and the updated sendMessage() method. right? In real world applications there will be tons of methods in the parent class. we created a SAPMessenger class for client 2 and just implemented the sendMessage() method with updated functionality. look at the test class shown in listing 4.out.out. System. if you noticed.13b Listing 4. SAPMessenger inherits the methods formatMessage() and sendMessage() from MainframeMessenger. it has two methods namely formatMes sage() and sendMessage().Chapter 4
Object Oriented Programming
Method Overwriting
public void sendMessage(String message) { formatMessage(message). we eliminated duplicating the formatMessage() method. Now. 1. 2. The updated sendMessage() method overwrites the inherited sendMessage() method.println("Message" + message + " sent to Mainframe"). This process of defining the exact same method in the child class that is in the parent class. Is this going to work? Yes it will. System. public class SAPMessenger extends MainframeMessenger { public void sendMessage(String message) { formatMessage(message). } }
Looking at the mainframe messenger class. This class is used by client 1.13c. } }
As you can see from the above code.println("Message" + message + " sent to SAP System"). The child class is now said to have overwritten the method in the parent class.

Message Formatted Message Hello sent to Mainframe
Using s1 reference.13c (OverwritingTest. and produces the following output
Message Formatted Message Hello sent to SAP System
Compile all the above 3 classes.println("Processing System. s1.out.Chapter 4
Object Oriented Programming
Method Overwriting
Listing 4. we clearly know that we eliminated writing duplicate methods in the class using inheritance and method overwriting. 4"). m1. and execute the test class to see the above results. 3"). By using m1 reference it invokes the method from the mainframe class which produces the following output.out.out. Consider a class called Processor with a method process() that processes four steps used by client 1 as shown below:
void process(){ System. it invokes the updated method in the SAP messenger class.println("Processing } Step Step Step Step 1").out. public class OverwritingTest { public static void main(String args[]) { MainframeMessenger m1 = new MainframeMessenger().java) Test class for client 1 and client 2
package objectconcepts.println("Processing System. } }
The above test class created both the messengers and invoked the sendMessage() methods.
Now my new client requires a process() method to process six steps as shown below
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. 2"). SAPMessenger s1 = new SAPMessenger().println("Processing System.sendMessage("Hello").sendMessage("Hello"). Let’s look at another classical example that also eliminates duplicate statements in the methods. In the above example.

create a child class of Processor class and overwrite the above method to include six steps.out. the best solution is have the child class process() method invoke the parent class process() method.out.println("Processing System.out. The syntax for invoking methods using super keyword is shown below: super. The situation is even worse if we have several hundred statements like in real world applications. the child class process() method will now have the duplicate statements from parent process() method plus additional statements. right? Though this is what we should do.out. 5").println("Processing System. In such situations.methodName(). The super keyword is used by child classes methods to invoke the parent class methods.println("Processing System. This is when we use the keyword super. 3").out. what we need to do is. 2").println("Processing System.println("Processing } Step Step Step Step Step Step 1").out. 3"). 6"). } public void process() { System.out.println("Processing Completed"). Now look at the code in listing 4.out.println("Processing Initiated").out. right?.14 that demonstrates the usage of super keyword and smartly eliminating the duplicate statements.println("Processing System. and just define the additional statements.println("Processing System.14a (Processor. 2").
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. Listing 4. public class Processor { public void begin() { System. 4").out.out. 4").println("Processing } public void stop() { System. Step Step Step Step 1").out.println("Processing System.Chapter 4
Object Oriented Programming
Method Overwriting
void process(){ System.println("Processing System.java) Client 1 code
package objectconcepts.
So.

When you overwrite a method.
Processing Processing Processing Processing Processing Processing Initiated Step 1 Step 2 Step 3 Step 4 Completed
When the execute() method is invoked using p2 reference. where the child class defines the exact same method as the one in parent class. The process() method first invokes the parent class method. :-) Hopefully I didn’t confuse you until now. Don’t ask me where the begin() and stop() methods came from.
Processing Processing Processing Processing Processing Processing Processing Processing Initiated Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Completed
This is all about method overwriting. comes back and executes the remaining two statements. The rule is pretty simple. Overwriting is related with 2 classes namely parent class and child class. when the execute() method is invoked using p1 reference. I am sure the examples thus far are pretty straightforward. This is opposite to overloading. You need to have this on your finger tip. argument list. the result will be. One of the primary goals of this book is not to create any confusion at
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. executes the four statements.Chapter 4
Object Oriented Programming
Method Overwriting
p2. right? Interview question: What is the difference between Overloading and Overwriting? Answer: Overloading is related with one class where more than one method in the same class shares the same name. } }
In the above test class. So. They are inherited from the parent class. process() and stop() methods in the AdvancedProcesssor class. it invokes the begin().execute(). No changes at all. ok. keywords and return type all must be the same. Overwrite a method in a class when some one needs a completely updated behavior in the existing class. So the result will be. the rule is that the method name. it internally invokes the begin(). process() and stop() methods from Processor class.

we already learned how to define a constructor and how the JVM invokes the constructors. read the same again. Trust me.
package objectconcepts.println("I am in Child int constructor"). public class Child extends Parent { public Child() { System.Chapter 4
Object Oriented Programming
Constructors Revisited
any point of time. } }
The code shown in listing 4.15a and 4. You know. } public Parent(int val) { System.out. skip it. } }
Listing 4.15b (Child.15b. public class Parent { public Parent() { System. Listing 4. Is there anything that we missed? Yes. Let’s see four cases here.15a (Parent.
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. but anywhere :-)
Constructors Revisited
It’s time to revisit constructors again. Consider the two classes shown in listing 4.println("I am in Parent default constructor").out.out.println("I am in Parent int constructor"). not just with this book. a small concept around constructors during inheritance.out.println("I am in Child default constructor").java) A typical child class with 2 constructors. If something is still confusing. We also know how to overload constructors.15 shows two classes namely Parent and Child each having a default constructor and a constructor that takes int as an argument. The important thing is we should always keep moving forward. } public Child(int val) { System.
package objectconcepts.java) A typical parent class with 2 constructors. You’ll be fine. If you still don’t understand.

the JVM invokes the constructor that takes int. So. So.
The above statement is creating a parent object with constructor taking an integer argument. Case 3: What will be the result of the following statement?
Child c1 = new Child(). This is pretty simple.
I am in Child default constructor
But the actual result will be
I am in Parent default constructor I am in Child default constructor
Stumped! How can this be possible? Take a look at the next case. So. Let’s look at next case Case 2: What will be the result of the following statement?
Parent p1 = new Parent(10). the JVM invokes the default constructor. The result will be. Case 4: What will be the result of the following statement?
Child c1 = new Child(10). Which ever constructor we used while creating the Parent class object. the same constructor is executed. like before we expect the result to be.
The above statement is creating a child object with default constructor. Let’s look at next case.
I am in Parent int constructor
No surprises until now. and the result will be.
I am in Parent default constructor
Good.
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.
The above statement is creating a parent object with default constructor.Chapter 4
Object Oriented Programming
Constructors Revisited
Case 1: What will be the result of the following statement?
Parent p1 = new Parent().

we expect the result to be. Here is the JVM’s rule of executing the constructors under inheritance. Listing 4.15c is the test class for the above code. Parent p2 = new Parent(10). With the above rule. Child c1 = new Child().
I am in Child int constructor
But the actual result will be
I am in Parent default constructor I am in Child int constructor
Once again stumped. Listing 4. when creating an object of that class. what the hell is going on here. JVM always executes all of its predecessors (all the parents in the inheritance chain) default constructors. Child c2 = new Child(10). So.Chapter 4
Object Oriented Programming
Constructors Revisited
The above statement is creating a child object with integer constructor. look at Case 3 and Case 4 again. You’ll no longer be stumped. } }
Compile all the above three classes. and execute the test class as shown below
C:/>JavaTraining>Chapter4>javac objectConcepts\*.java C:/>JavaTraining>Chapter4>java objectConcepts. public class ConstructorTest { public static void main(String args[]) { Parent p1 = new Parent(). Rule: Before executing any child class constructor. It’s very important to remember this point. Thumb Rule: If a class is participating in inheritance.15c (ConstructorTest.ConstructorTest
The output of the above program will be
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. JVM always executes parent class default constructor.java) Test class for constructors during inheritance. Shoot. No need to get frustrated guys.
package objectconcepts.

16. super(20) invokes the parent class integer constructor and so on. As part of continuous enhancements. and decides to add more initialization steps on top of existing initialization process hoping to improve the performance. we saw how constructors invoked other constructors of the same class using this keyword. it decided to release version 6 after 1 year and fix some of the bugs in the previous release. But in real world applications. Let’s say the version 5 database is suffering from serious performance issues.Chapter 4
Object Oriented Programming
Constructors Revisited
I I I I I I
am am am am am am
in in in in in in
Parent default constructor Parent int constructor Parent default constructor Child default constructor Parent default constructor Child int constructor
If you look at the previous example. all the child class constructors are only invoking the parent class default constructor. Today it released version 5 of the database which has certain initialization. Look at the code in listing 4. After serious research the company attributes the problem due to bad initialization of the database. the statement super() invokes the parent class default constructor. to eliminate duplicate initialization statements. it should use super keyword.16a (ExtremeV5. public class ExtremeV5 {
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. we use super() constructor calls to avoid duplicate initialization code in child classes. Now that we know how to use super to invoke the parent class constructors.
package objectconcepts. Just like in one of the example we used super to invoke a parent class method to eliminate the duplicate statements. Let’s look at more practical application. the question that comes to my mind is when and why a child class should invoke the parent class constructor? Again the reasoning is simple. Extreme Solutions Inc is a company that develops a database called “ExtremeDB”. Listing 4. Let’s now write a program to implement this scenario.java) Version 5 code of the database. If you recall one of the previous examples. for a child class to explicitly invoke any of the parent class constructors. For instance. Similarly. it would be more beneficial if the child class constructors have the ability to invoke which ever parent constructor it want to rather than JVM always invoking the default constructor as shown above.

i++) { System. ok.16c (ExtremeTest.
The above statement executes all the initialization statements in ExtremeV5 parent class constructor.println("Executed Statements").println(" Initializing Channel " + i). Once completed. Now. Listing 4.Chapter 4
Object Oriented Programming
Constructors Revisited
public ExtremeV5(int counter) { for (int i = 1. Listing 4. } } public void execute() { System. public class ExtremeV6 extends ExtremeV5 { public ExtremeV6(int counter) { super(counter). } } }
As you can see from the above code.java) Test class for version 5 and version 6 databases.16b.
package objectconcepts. the control comes back to the ExtremeV6 constructor and executes the rest of the initialization statements. } }
If you look at the ExtremeV5 class. we created ExtremeV6 class whose constructor explicitly invokes the parent class constructor as shown below:
super(count).
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.out.16b (ExtremeV6. i++) { System. i <= counter.
package objectconcepts. // Additional Initialization for (int i = 1. This constructor has some initialization. take a look at the code in listing 4. i <= counter.out.println(" Initializing Partition " + i). it defines a constructor that takes an integer argument.out. Listing 4.16c shows the test class for the above two classes.java) Version 6 code of the database.

Now. There is one rule here shown below.println("Using Oracle Database V6"). and a single child constructor cannot invoke more than one parent constructor. With this. v6. v5. This is how we use super keyword to invoke the parent class constructors.println("Using Oracle Database V5"). ExtremeV5 v5 = new ExtremeV5(4). the version 6 of the database used the initialization from version 5 followed by its own initialization statements. Rule: The super statement must be the first statement in the constructor. System.
Using Oracle Database V5 Initializing Partition 1 Initializing Partition 2 Initializing Partition 3 Initializing Partition 4 Executed Statements Using Oracle Database V6 Initializing Partition 1 Initializing Partition 2 Initializing Partition 3 Initializing Partition 4 Initializing Channel 1 Initializing Channel 2 Initializing Channel 3 Initializing Channel 4 Executed Statements
As you can see from the above result.out. we now know everything about constructors. it’s time to learn something about abstract methods. classes and their significance. ExtremeV6 v6 = new ExtremeV6(4).execute(). Good. The output of the program will be.Chapter 4
Object Oriented Programming
Constructors Revisited
public class ExtremeTest { public static void main(String args[]) { System. Let’s see what these are.out.execute().
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. } }
The test class creates objects of both the classes by passing the argument value 4 to their constructors and invokes the execute() method. Abstract classes play a very important role in object oriented design.

one of the common things we try to do is define a generalized class with method definitions. Define the method and put a semicolon after the method signature. The generalized class is like a definition class that only knows what methods a class must have.Chapter 4
Object Oriented Programming
Abstract Methods
Abstract Methods
In object oriented design. Note that we again have to use the abstract keyword in the class declaration. a class besides having abstract methods can also have non-abstract methods (ones that are implemented).
public abstract void process(). but doesn’t know how to implement the methods (define the body). following is an abstract process() method. Following are the two simple rules to define an abstract method. As an example. This is why we also call abstract class as a partially implemented class. The syntax for declaring abstract methods is shown below abstract <return type> methodName(argument list). 1. and let other classes implement the methods in their own way. Such methods are said to be abstract as they don’t have any implementation. the keyword we need to use is abstract. what is an abstract class? If a class has at least one abstract method. and terminated with a semicolon without writing any statements.
Abstract classes
Now. To define an abstract method. However. the class is said to be abstract and must be declared abstract as shown below:
public abstract class Test
A class can have any number of abstract methods.
All we did is included the abstract keyword in the method declaration. 2. Look at following 2 simple rules with abstract classes:
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. No curly braces {}. Declare the method as abstract using abstract keyword.

Now the child class becomes fully implemented class and we should use this class for creating objects. Car manufacturers will then use my design and build the car with their own technology and features. I am a car designer. it must implement all the methods. If you have a situation where you know what methods a class must have. Listing 4. they should only implement other specific components. A class must be abstract if it has at least one abstract method.17a (Car. create a child class and have it implement all the abstract methods.Chapter 4
Object Oriented Programming
Abstract classes
1. So the JVM itself forbids creating objects. I create the design and sell it to various car manufacturers. there should be one or more child classes of the abstract class to implement all the abstract methods. I know there will be some things that are common to every car. right? To make the class useful. This makes sense because. If this is the case. addEngine(). Let’s look at an example and see how abstract methods are used.17 shows the code for this scenario using abstract classes and child classes. Here is the de facto rule. Then somebody who knows how to implement abstract methods should write a child class of the abstract class. it says we are not allowed to create objects of that class. If the class is declared abstract. As a designer. 2. Every child class can implement the abstract methods in its own way. the class is partial. Rule 2 is interesting.
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. but cannot implement some of the methods. public abstract class Car { public public public public public public public abstract abstract abstract abstract abstract abstract abstract void void void void void void void assembleTyres(). shiftGears(). If we have an abstract class. we cannot create objects of that class using new keyword. drive(). Make sense? Yes it does. If a class is abstract. then use an abstract class. and we don't want to create partial objects.
package objectconcepts. Listing 4. what we therefore need to do is. addSeats(). startCar(). addCover(). But the rule is. then this class is fit for nothing even though it has few implemented methods. Instead.java) Abstract car class. like the way the car should be driven and the manufacturers should never implement them. and provide the implementation for those some unimplemented methods.

} }
As you can see from the above code.
Assembled GoodYear tyres Added Metallic Cover Added 500 HP engine Added 2 Leather seats Car started with AutoStart Has Automatic Transmission Driving at 80 mi/hr
In Java.
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.out. This is how we use abstract classes and methods. public class CarTest { public static void main(String args[]) { Honda h = new Honda(). Therefore. the Honda class is created from the Car class and implemented all the abstract methods as it knows how to implement them.testDrive().Chapter 4
Object Oriented Programming
Abstract classes
public void shiftGears() { System. Now look at test class shown in listing 4. it has all the methods implemented and we can create objects of this class.out. } public void drive() { System.println("Has Automatic Transmission").println("Driving at 80 mi/hr").
package objectconcepts. This method internally invokes all the methods and generates the following output. This class inherits the testDrive() method. Using abstract classes is very common in real world applications as it allows multiple implementations of certain behavior. } }
The test class simply created Honda object and invoked the testDrive() method. h. Variables cannot be abstract.17c (CarTest. only methods and classes can be abstract. Likewise.17c. The next topic is final methods and final classes. you can create another class like Toyota and implement the same methods but in a different way. Listing 4.java) Test class for invoking methods.

Honda class can overwrite it.Chapter 4
Object Oriented Programming
Final methods
Final methods
Like abstract methods and abstract classes. The Car class need not live at the mercy of Honda class anymore. If a variable is final. They must adhere to the design. But now that we know what a final can do. In the previous example. This is like enforcing the rule. then simply declare the method as final as shown below
public final void testDrive(). you need not declare the class as final if it has final methods. Now I am 100% sure that no car manufacturer will ever dare to overwrite the method. If one of the methods in a class is final. So. if a class is declared as final. it cannot have any child classes. A variable can be final. and the Honda class can’t overwrite it. the testDrive() method in the Car class is not final. This is how we use final with methods. So the Car class is at the mercy of Honda class not to overwrite it. Unlike abstract. methods and classes can also be final. JVM will not allow any of the child classes to overwrite the method.
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.
When the method is declared as above. However. its value cannot be changed once assigned as shown below:
final int MAX_QTY = 100. But it didn't. you can make the class final at any time to gain one advantage. The advantage is preventing inheritance of that class. So. The usage of final with methods and classes is very simple. without any hesitation attach the final keyword to the testDrive() method.
Final class
Let me ask you a question here. should the class be declared final? Nope. If you want a certain method in a class not to be overwritten in the child class.
Final variables
Can a variable be final? Yes. Following is how a final class looks like:
public final class Test { }
Trying to create a child class of the above class using extends keyword will cause a compilation error. This is done using the final keyword.

If a class is declared final. What if all methods in the class are abstract? Good guess. The class is said to be 100% abstract. JVM knows the methods are implicitly abstract. it cannot be inherited . Here are the differences between final and abstract keywords. This means. The moment we use an interface.Used with Class. This is why we call final variables as CONSTANTS. Not with variable. Now the last concept in this chapter. method and variable . an abstract class is a partially implemented class.If a variable is declared final. . it cannot be overwritten in the Child class.If a method is declared final.If a method is abstract it must be overwritten in the Child class. . As I said before. Java has provided us with a special entity to use whenever all the methods are known to be abstract. The good thing with interfaces is that. It is a CONSTANT abstract . Interfaces. Let’s summarize the usage of final and abstract. final . .
Interfaces
Let’s recall the notion of abstract classes here for a second. An interface is nothing but an entity whose methods are all abstract by default. final keyword and abstract keyword work totally opposite.Chapter 4
Object Oriented Programming
Interfaces
Trying to alter the value of MAX_QTY will cause a compilation error. some of its methods are well and truly implemented and some of them are abstract. you need not declare the methods using abstract keyword anymore.If a class is abstract. then it must be inherited (sub classed) before using it. This is 100% guaranteed interview question. This special entity is what we call as interface. Following is the syntax for declaring an interface with methods.
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. right? Keeping this in mind.Used with Class and method only. Let me ask you a question. its value cannot be changed. and see the differences between the two.
Difference between abstract and final
If you noticed carefully.

I know how to design a database. We use them by creating objects of databases and invoking the methods. we need to write a class that implements the interface. Method 2 (). . Method N(). } The keyword used to declare an interface is interface. Let is look at a classical Database example using interfaces. MySQL etc) These guys use the above design (Database interface) and provide implementation of all the elements (methods) Users: (Test class) These are the users of the database who use the above created databases like Oracle and MySQL. So he creates a Database interface with all the methods a database must have. Problem solved.
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. if we have an interface. I give my design to those people. Designer This guy knows what elements a database must have and cannot implement any of these elements (methods). Database Creators: (Oracle. We need to write a class that implements all the methods in the interface. So. Otherwise. However.Chapter 4
Object Oriented Programming
Interfaces
public interface <interfacename> { Method 1 (). Good. So the rule is. Once we defined an interface with all the methods. Cool. Problem: As a designer for databases. who will then use their expertise to implement the database. . the existence of an interface doesn’t make any sense unless someone implements its methods. But there are good number of people out there who knows how to implement database based on the design. it does. Take a look at the code in listing 4. I don't know how to create a working database based on this design. Let's implement this scenario. are we done? No. Make sense? Yes.18. such as what elements a database must have an all that good stuff.

For a class to implement an interface. System. executeQuery().println("SQL Executed"). } public void executeQuery() { System. Notice that we didn’t use any abstract keyword here as they are implicitly abstract by default. public class Oracle implements Database { static { System.println("Oracle Shutdown Completed"). } private void clearCache() { System.println("Cache Cleared"). shutDown().out.println("Got the connection from Oracle DB").18b (Oracle.
package objectconcepts. } public void createStatement() { System.println("Oracle DB Initialized"). we need to write a class to implement the above interface.18b. we defined a class named Oracle that implements all the methods in the interface.out. createStatement(). Since the above interface is of no use alone.out.java) Class implementing the database interface.
The above Database interface defined four methods without any implementation.out. Listing 4. } }
As you can see the above code.Chapter 4
Object Oriented Programming
Interfaces
Listing 4.out.
package objectconcepts.18a (Database. public interface Database { public public public public } void void void void getConnection().println("Created Oracle Statement").out.java) Interface defining the methods. } public void shutDown() { System. } public void getConnection() { clearCache(). it must use the
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. Now look at the implementation class shown in listing 4.

In our case. That’s it.
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. At the bare minimum it must implement all the methods of the interface.18c (OracleUser.createStatement(). the Oracle class also implemented its own private method clearCache(). db.java) Test class for using Oracle database. Listing 4. Listing 4. given an interface.shutDown(). // Invoke the methods. We now have a class with all the functionality implemented. we can write multiple implementation classes for the same interface. See the following class declaration. No restriction.
package objectconcepts. } }
The test class is pretty straight forward. we can create another class from the same interface and implement the methods in its own way as shown below:
public class MySQL implements Database{ // implement all the methods }
The moral of the story is. ok. and invoked the methods.
public class Oracle implements Database
This class besides implementing the methods in the interface can also have its own methods.executeQuery().18c shows the test class.which it uses as a helper method while implementing the interface methods. public class OracleUser { public static void main(String args[]) { // Create the implementaton object Oracle db = new Oracle(). Just like we created oracle class.getConnection(). and we can now create the objects of this class to invoke the methods. db. It simply created an instance of Oracle class.Chapter 4
Object Oriented Programming
Interfaces
keyword implements just like we used extends keyword when creating a child class. db. db.

Listing 4. C are three different interfaces. Let me ask you a question here. If you have a situation where you know what methods a class must have.Chapter 4
Object Oriented Programming
Interfaces
When do we use abstract classes and interfaces?
Good question. let’s look at the next example that uses multiple interfaces. This is a very important interview question. You must then write a child class that extends the abstract class. You must then write a class that implements the interface.java) Database interface.” True/False The answer is false.C
where A. public void executeQuery().
package objectconcepts. but cannot implement any of the methods. in the former "some" methods are unimplemented. and provide the implementation for all the methods. Is the following statement true or false? “Like a child class cannot inherit from more than one parent class. use this class to create objects. a class cannot implement more than one interface. it must implement all the methods from all the interfaces. then use an interface. So. Make sense? Yes it does. The only difference between an abstract class and interface is.B. So learn it twice. if a class implements more than one interface.19.19a (Database. See listing 4. B. Finally. The syntax is as shown below:
public class Test implements A. }
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. and provide the implementation for those some methods that are not implemented. but cannot implement some of the methods. The only rule is that. If you have a situation where you know what methods a class must have. public interface XYZDatabase { public void getConnection(). then use an abstract class. and in later "all" methods are unimplemented. Classes can implements ‘n’ number of different interfaces. Then use this class to create objects.

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.19c (XYZDatabase.
package objectconcepts. db.java) Class implementing both the interfaces. public interface Driver { public void loadDriver(). XYZDatabase { public void loadDriver() { System. Once we have this class.
package objectconcepts.19b (Driver. } }
The above class implements both the interfaces.getConnection(). } public void executeQuery() { System.19c that implements both the above interfaces.out. public class XYZDatabase1 implements Driver.Chapter 4
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Interfaces
Listing 4. public class XYZDatabaseUser { public static void main(String args[]) { XYZDatabase db = new XYZDatabase().java) Driver interface.out.println("Executed Query").println("XYZ Driver loaded"). } public void getConnection() { System. Notice that we implemented all the three methods from the two interfaces. Listing 4. These are pretty straight forward.java) Test class invoking all the methods in the interface.
package objectconcepts. }
As you can see the above code.19d (XYZDatabaseUser. Listing 4. Now look at the code in listing 4.println("Got Connection"). db.loadDriver(). we can write a test class as shown below and invoke its methods. the two interfaces defined three methods.out.

Indicates whether some other object is "equal to" this one. Called by the garbage collector on an object when garbage
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. right?. there is an ultimate parent called Object class. we have seen how to create a child class from parent classes using extends keyword. Likewise in Java. } }
All we did in the above test class is invoked the methods in the implementation class.1 lists the methods in the Object class Table 4. right? Let me ask you a question. This is the ultimate parent of any inheritance hierarchy. Compile and execute the test class to see the following result. So.Chapter 4
Object Oriented Programming
The Object Class
db. If a class doesn’t extend from any class. read them twice. Surprised? We all know we have parents because we physically see them. then we can be 100% sure that there is a parent class. This logically means.executeQuery(). does this mean it has no parent? Is it an orphan class? The answer is No. There will hardly be any real world Java applications that ever use interfaces and abstract classes. Before we verify the this fact.
XYZ Driver loaded Got Connection Executed Query Executed Query
This is how you need to use interfaces.
The Object Class
When we learned about inheritance. If a class doesn’t use extends keyword. let’s first look at what methods the Object class has. if a class uses extends keyword in its declaration.executeQuery(). there is one ultimate parent whom we cannot see in our life who is none other than ‘God’. a very powerful concept in Object Oriented design.1 Object class API Method Object clone() boolean equals(Object o1) void finalize() Description Creates and returns a copy of this object. based on inheritance rule it must inherit the methods from the Object class. But. This is why I say Java is close to real life. Table 4. db. By saying that any class is a child of Object class if it doesn’t use the extends keyword. it’s an implicit child of Object class.

Listing 4. Causes current thread to wait until another thread invokes it.println(number). } }
Listing 4. System. Wakes up all threads that are waiting on this object's monitor. let’s execute the following programs. Returns a hash code value for the object. to verify the fact that these methods are indeed inherited into any class that doesn’t extend from any class.
Out of all the above methods. // Invoke the inherited method String str = ph.
package objectconcepts.20b (PhoneTest.java) Test class.out.java) Simple class. Returns the runtime class of an object.out.Chapter 4
Object Oriented Programming
The Object Class
Class getClass() int hashcode() void notify() void notifyAll() String toString() void wait()
collection determines that there are no more references to the object.20a (PhoneNumber. Wakes up a single thread that is waiting on this object's monitor. } }
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. String number = ph. System. public class PhoneNumber { public String getNumber() { return "12345678". public class PhoneTest { public static void main(String args[]) { PhoneNumber ph = new PhoneNumber(). So.
package objectconcepts. Returns a string representation of the object. the only method that we are interested and most widely used is the toString() method even though all the above methods are inherited.getNumber().toString().println(str).

JVM automatically invokes the constructor at the time of creating object using new keyword. and therefore the class will have all the methods from Object class plus its own method. you’ll not see any compilation errors which proves that the method toString() is inherited from the Object class. An Object is an instance of class.Chapter 4
Object Oriented Programming
Summary
If you look at the PhoneNumber class. One such inherited method from the Object class is the toString() method which returns a String value of the object. Objects can be passed as arguments to methods. A Constructor is like a special method whose name is same as class name and doesn’t have a return type. If you compile this class. Method overloading allows more than one method to share the same name in a single class. The rules are same. Encapsulation hides the details and presents the essentials of an object. Constructors can also be overloaded like methods. it just defined the getNumber() method. but not the Object itself. the test class created the object referenced by ph and invoked both getNumber() as well as toString() invisible method. This is achieved by using the keywords private and public. This completes all the concepts we need to know to work with objects. The keyword static differentiates between Class level methods and Object level methods in the sense Object level methods are non-static and class level methods are static. In this case. A single class can have ‘n’ number of objects of the same class. To verify this fact. The output will be some funky alphanumeric for the first one. we pass the reference (pointer) of the object.
Summary
Object Oriented Programming is “Programming using Objects”. All object methods can access static class methods as shared methods. and the phone number for the second like as shown below:
abcd1234h@PhoneNumber 12345678
Just remember that Object class is the ultimate parent of any class that doesn’t extend from any other class. just a static block is used for class level initialization. Since this class didn’t extend any class.
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. but the reverse is not true. Objects interact with each other by invoking each others methods. A Constructor is used for object initialization. let’s summarize all we learned until now in this chapter. So. it inherits from the Object class.

super(10) etc. then its value cannot be changed. If a class is final.Chapter 4
Object Oriented Programming
Summary
For one constructor to invoke another constructor. If a constructor is not defined in a class. JVM provides with a default constructor. abstract class is also called as partially implemented class. But a class can never have more than one parent class. If a class has any abstract methods. Inheritance eliminates the code duplication. it can use super keyword. An abstract class however can also have some methods implemented. It’s a Constant. We then have to use this class to create objects. JVM always invokes the parent class default constructor first before invoking the child constructor. The most important method in the Object class is toString() method. Multiple inheritance is strictly not allowed in Java. we use this keyword and pass arguments within the parenthesis. the class must also be declared abstract. The ultimate parent of any class is the Object class. Using this. we cannot create child classes from it. Ex: super(). This is why. When a child class object is created with any child class constructor. and implicitly has all the methods in the Object class. A class must implement the interface and define the body for all the methods in the interface. a child class can inherit all the non-private members of the parent class. then it cannot be overwritten in the child class. If a class doesn’t use extend keyword in it declaration. If a final keyword is used with a method. Method Overwriting allows a child class to define the exact same method as the one in the parent class. this(10. Inheritance allows one class to inherit the properties of another class. If a class is abstract or partial. An interface is an entity whose all methods are implicitly abstract.
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. ‘a’) etc. If a variable is final. it is implicitly a child of Object class. For the child class constructor to explicitly invoke an appropriate parent class constructor. we cannot create objects of its class using new keyword. A default constructor is an empty constructor with no arguments and no body. In such cases. We need to create a child class from abstract class and implement all the abstract methods. A class can implement any number of interfaces. Ex: this(10). super keyword is used by a child class to explicitly invoke the parent class methods. the method must be declared using abstract keyword. An abstract method is a method that doesn’t have any body.

display().display(). Which of the following keyword is used for distinguishing class methods and object methods a) void b) static 3. Which of the following is a legal way of invoking an Object level method? a) Test t = new Test(). which of the following is a valid constructor? b) int Test() c) Test() 8. b) Test. Which of the following is an object level method? a) void display(). b) static void display(). 5. 6. Which of the following keyword is used for invoking constructors of same
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.Chapter 4
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Time to play 50-50
Time to play 50-50
1. Which of the following statement creates an Object? a) Test t = new Test(). Which of the following statement is true a) Class is an instance of Object b) Object is an instance of Class 2. t. Given a class Test. b) Test t = Test(). 4. Which of the following are valid overload methods? a) void test() and int test() b) void test() and int test(int k) 7.

Chapter 4
Object Oriented Programming

Time to play 50-50

class? a) super b) this 9. Which of the following keywords us used for hiding the details of Object? a) public b) private 10. A simple class with variables and getter methods and setter methods is called as a) Java Bean or POJO b) Enterprise Java Bean 11. Which of following is a valid is true for overwriting a method? a) Same name, same return type and same arguments b) Same name, same return type, different arguments 12. Which of the following is a valid abstract method? a) abstract void display(){ } b) abstract void display(); 13. Which of the following is a valid class? a) class Test{ abstract void display(); } b) abstract class Test{ abstract void display(); } 14. Which of the following is a valid interface? a) interface Test{ void display() { Systen.out.println(“Test”); } }

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Interview Questions

b) interface Test{ void display(); } 15. Which of the following is a valid class declaration a) public class Test extends Test1,Test2 b) public class Test implements Test1,Test2 I am sure you cracked all of the above in a minute. The only reason I want to give you just two options is that you don’t have to think out of the blue. More the options, more the thoughts, more the questions, and more the confusion which is what I intend to avoid. Keeping life simple and easy, way to live.

Interview Questions
Question: Can you tell me what are the important OOPS concepts? Answer: The most important OOPS concepts are Encapsulation, Inheritance, and Polymorphism. Encapsulation hides the details and presents the essentials of an object using private and public keywords. Inheritance is a technique which allows one class to inherit the variables and methods of another class. One is called the Parent class and the other is called the Child class. A child class can only inherit the non-private members of the parent class only. Polymorphism allows methods in a class to be represented in multiple forms. This is done through Overloading and Overwriting. Question: What is Overloading and Overwriting ? Answer: Please refer to the Summary section. Question: What is the difference between abstract and final keyword? Answer: Please refer to page 114. Question: What is the difference between this and super

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Answer: this keyword is used for invoking the methods, variables and constructors of the same class. super keyword is used by the child class to invoke the variables, methods and constructors of parent class. That’s all we need to know in this chapter. This completes all the concepts in Object Oriented Programming. Wow. Let me tell you one thing. Though there are plenty of concepts, all of them are very simple and easy to understand. Moreover, the terminology is almost what we speak in our day to day life, which is why Object Oriented Programming is so fun and interesting. I am sure you thoroughly enjoyed this chapter and more importantly with passion. If possible read this chapter twice before going to next chapter. Java programming is all about creating objects and invoking methods. Let’s move one more step forward.

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Chapter 5
Referential Polymorphism

Chapter Goals

Chapter 5
Referential Polymorphism
This chapter teaches you on how to use parent class and interface references. By the end of this small chapter, you will be able to know how and when to use inheritance and interfaces in real world applications. The ideas and examples in this chapter are totally based on what we learned in the previous chapter. We are not trying to reinvent the wheel here, ok. Understanding the concepts in this chapter is utmost important since most of the real world Java applications are based on Object Oriented Design whose primary weapons are inheritance and interfaces.

Chapter Goals
Understanding parent class references Understanding Interface references How to restrict access to methods using Inheritance How to restrict access to methods using Interfaces

Environment Setup
All the programs in this chapter should be stored in the following directory.
C:/JavaTraining/Chapter5/objectRefs objectRefs is a package in which we will store the programs. To compile the programs, move to the following root directory and get ready. C:/JavaTraining/Chapter5

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Introduction

Introduction
In Java, it’s all about different objects invoking methods in different objects. Methods are like sophisticated operations and sometimes it’s important that we restrict access to some of the operations to certain objects and allow the same to some other group of objects. In the previous chapter, we learned something called encapsulation where access to methods is restricted using private keyword. However, the private keyword restricts access to all the outside objects which is not what we wanted. We wanted to restrict access to some objects and allow access to some others. So, using private keyword is definitely not the solution. The solution to this problem is using inheritance and interfaces. In the previous chapter we learned how inheritance and interfaces allows us to have multiple implementations of the same methods, and at the same time, how to eliminate duplicate code. This is one side of inheritance and interfaces. The other side of it is what the most powerful one is, controlling access to methods, one of the key application in Object Oriented Design. Let’s see how this is achieved. We already know how to create objects from a parent class and a child class, and what methods we can invoke on the same. Just as a recap, we’ll see one example and invoke the methods in both the classes. Take a look at the code in listing 5.1. Listing 5.1a (Parent.java) A simple parent class.
package objectrefs; public class Parent { public void formatMessage() { System.out.println("Message Formatted"); } public void sendMessage() { } } System.out.println("Message Sent");

As you can see from the above code, the Parent class defined two methods namely formatMessage() and sendMessage(). Its child class is shown in listing 5.1b.

As you can see from the above code, the child class overwrote the formatMessage() and defined new method namely recieveMessage(). Now take a look at the following two simple cases. Case 1: A parent object is created as shown below.
Parent p = new Parent();

Using reference variable ‘p’, we can only invoke the methods in parent class as shown below:
p.formatMessage(); p.sendMessage();

Case 2: A child object is created as shown below.
Child c = new Child();

Using reference variable ‘c’, we can only invoke the methods in child class as shown below:
c.formatMessage(); // Overwritten method c.sendMessage(); // Inherited from parent c.recieveMessage(); // Its own method

No confusions, right? We simply invoked the respective methods on each bject. This is what we learned in the previous chapter. Good.

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Loose Coupling

Loose Coupling
One of the primary goals in Object Oriented Design is to design the system in such a way that various objects are loosely coupled with each other to increase the flexibility of the overall system. To understand loose coupling, consider the following example. Let’s say my company want to build a search engine that uses web tool to search for resources over the web. To develop this system, I need two classes namely SearchEngine and WebTool. Look at the following code in listing 5.2 for both the classes. Listing 5.2a (WebTool.java) A simple web tool class.
package objectrefs; public class WebTool { // Search method public void search() { System.out.println("Found 10 results in Web"); } }

I am sure you know how the above code works. The WebTool class defined a search() method that displays the search results. This is the method our search engine class invokes to display the search results. Now look at the search engine class in listing 5.2b. Listing 5.2b (SearchEngine.java) A simple search engine class.
package objectrefs; public class SearchEngine { // This method takes the tool as parameter and peforms // the search using it. public void performSearch(WebTool tool) { tool.search(); } }

As you can see from the above code listing, the SearchEngine class defines a method performSearch() that takes the WebTool object as a parameter, and invokes the

The test class created a WebTool object and passed it as a parameter to perform Search() method of the search engine object as shown below:
engine.performSearch(tool);

The above method internally invokes the search() method in the WebTool class. Executing the test class will give you the following result.
Found 10 results in Web

So far so good. Let’s say my client who is using the above code for searching comes to me and say, “Hey! Right now you are searching only using WebTool which is good. Sometimes we also want to search using another tool called ImageTool to search for images”. To fulfill this requirement, we need to implement the following two changes: 1. Create a class called ImageTool just like WebTool as shown below:
package objectRefs; public class ImageTool{ public void search(){ System.out.println("Found 10 Image results"); } }

Using the above SearchEngine class, my client will then be able to search using either tool. So, every time my client comes with a new search tool, I need to add a new search() method in the search engine class, right? We can say this class as a tightly-coupled class, since we need to update it every time our client comes with a new requirement, ok. This is not a good design. So the question is, what is the best design for the above class? To get to the best design, our goal is two fold: a) Eliminate the tight coupling and b) Use just one generic performSearch() method for all the searches. Trust me, we will definitely reach our design goal, but before that we need to know something about parent class and child class references.

Parent class referencing the Child Object
In Java, a child class object can be referenced by a parent class variable as shown below: SearchTool p = new WebTool(); The above statement is legal only if WebTool is a child class of SearchTool. Let me ask you a question here. Using the reference variable ‘p’, which methods can we access? a) All methods in SearchTool class b) All methods in WebTool class I am sorry to say that both are wrong. Here is the rule. Using reference ‘p’, we can invoke,

With the above two classes, when we create a child object as shown below,
SearchTool tool = new WebTool();

Rule 1 says “Only the methods in parent class”. So, the only methods we can invoke using ‘tool’ reference are format() and search() as shown below.
tool.format(); tool.search();

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Parent class referencing the Child Object

Since, format() method is not overwritten in the child class, it invokes this method from the parent (Rule 1.1) . The result is
In Parent Format

Since search() method is overwritten in the child class, it invokes this method from the child class (Rule 1.2) . So the result of this invocation will be
In Child Search

Here is a simple technique JVM uses to invoke the methods when a child object is referenced with a parent class variable. It first goes to parent class method and checks if the method is overwritten in the child class. If so, it invokes the child class method. Otherwise, it invokes the parent class method. This should take away your confusion. From the above statement, if we don’t override any methods in the child class, then all the methods will be invoked from the parent class, right? In such cases, following two statements are identical:
Parent p = new Parent(); and Parent p = new Child();

As you can see from the above code, we created an abstract class called SearchTool with an abstract search() method. Any search tool who knows how to search must extend the abstract class and implement the search() method. See listing 5.4b in which we create a child class of this class. Listing 5.4b (WebTool.java) Class extending the abstract class.

To understand the above code, look at the two snippets of code shown below:

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Parent class referencing the Child Object

engine.performSearch(new WebTool());

public void performSearch(SearchTool tool) { tool.search(); }

As you can see from the above, the WebTool object is passed into the SearchTool leading us to the following assignment.
SearchTool tool = new WebTool();

So, when the perfomSearch() method invokes the search() method using its argument tool reference, it actually invokes the method in the WebTool object passed, right? This is because the search() method is overwritten in the WebTool class and according to our rule, overwritten methods are invoked from the child class. Compile and execute the above classes, and the output will be,
Found 10 results in Web

With the above code, did we eliminate tight coupling? Yes we did. To prove this, let’s fulfill our client’s requirement who also want to use ImageTool to search for images. With the above code, all we need to do is create this class just like the WebTool class as shown below: Listing 5.4e (ImageTool.java) New search tool class.
package objectrefs; public class ImageTool extends EnhancedSearchTool { public void search() { System.out.println("Found 10 Image results"); } }

Trust me; you don’t have to change a single piece of line in the SearchEngine class. You don’t have to add any new method like we did in example 5.2 (Page 133). If my client needs to use this tool, all he has to do is simply pass the object of ImageTool class as shown in listing 5.4f.

Our perfomSearch() method in the SearchEngine class is now 100% generic. By changing the argument to parent class ‘SearchTool’, it can invoke the search() methods on any child search tools (WebTool, ImageTool etc). Now we should agree that our engine class is loosely coupled with the WebTool and ImageTool classes. Even if we update the WebTool code or completely stop using it, we still don’t have to modify our search engine class. You agree with me? The funda here is using the parent class references as method arguments instead of child class references. This is the trick. In most of the real world applications, the arguments of a method will always be a parent class references, and who ever invokes the method, should always pass the child objects. Fig 5.1 shows the design of our search engine class.

performSearch ( SearchTool tool)

search()

search()

WebTool

ImageTool

Fig 5.1 Parent class referencing child objects

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searchOracleDB().println("Found 5 results in LDAP").println("Found 10 Image results").5a (PremierSearchTool.
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. Listing 5.out.out.
The above requirement is very simple to implement. } public void searchOracleDB() { System. searchMySqlDB().Chapter 5
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Accessing methods using Inheritance
As I said before that we can restrict access to methods using inheritance and interfaces.out. All we need to do is write a class that implements all the above methods. To clearly understand the concept. let’s write a utility class that will return the above search tool to Client A as shown in listing 5. let’s now look a more practical example and see different cases for accessing methods in a class.5a.println("Found 10 Web results").searchImages(). } public void searchMySqlDB() { System. } public void searchImages() { System.java) Case 1 search tool
package objectrefs.println("Found 5 results in Novel").out. right? So.out.println("Found 2 results in Oracle DB"). Case 1: Client A needs a search tool with the following 6 search methods
searchWeb().out. public class PremierSearchTool { public void searchWeb() { System. searchLdap().searchNovel(). let’s first look at this using inheritance. } }
Now. } public void searchLdap() { System. } public void searchNovel() { System.5b.println("Found 3 results in MySQL DB"). our class will look as shown in 5.

package objectrefs. tool. public class SearchTools { public static PremierSearchTool getClientASearchTool() { // Create and return the search tool for client A PremierSearchTool p = new PremierSearchTool(). tool.
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Listing 5.5c (ClientASearchToolTest.getClientASearchTool().searchImages().searchMySqlDB(). } }
Compile and execute the above test class to see the following result.
package objectrefs. // Invoke the search methods tool. return p.
Found Found Found Found Found Found 10 Web results 10 Image results 5 results in Novel 5 results in LDAP 2 results in Oracle DB 3 results in MySQL DB
Now let’s look at another requirement.java) Client A search code. public class ClientASearchToolTest { public static void main(String args[]) { PremierSearchTool tool = SearchTools. tool.searchWeb().searchNovel(). His code will therefore look something like the one shown in listing 5. tool.java) Utility class to return the search tools for clients.searchOracleDB(). Note that the above method is declared static (class level) so that client code can call the method directly using class name.searchLdap(). Listing 5. } }
Client A can use the above class and invoke the method getClientASearchTool() to get his search tool.5c.5b (SearchTools.

the SuperiorSearchTool now has 7 methods with 4 old search methods from parent and 2 updated methods and 1 new method. right? This is nothing but reusing the code using inheritance. So. Hence this is really a superior search tool. public class SuperiorSearchTool extends PremierSearchTool { // Overwritten methods public void searchNovel() { System.println("Found 9 results in Libraries"). So. create a child class from the above PremierSearchTool class. Overwrote the two methods for novel search and ldap search Added one new search method for library search.println("Performing enhanced Novel Search").5d (SuperiorSearchTool.out. System. what we can do is.java) Client B search tool. System. } // New search Method public void searchLibraries() { } } System.5d.out.Chapter 5
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Case 2: Client B wants a superior search tool that has enhanced searching in Novel and Ldap.out. all we did is created SuperiorSearchTool class from PremierSearchTool class and. overwrite searchNovel() and searchLdap() methods. my search tool class for this requirement will be as shown in listing 5.
As you can see from the above code.
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.out. and also a new search for searching libraries.out. My utility class will return this search tool to Client B as shown below in bold.println("Found 15 results in LDAP"). and create a new method searchLibraries(). All other searches must remain the same. } public void searchLdap() { System.
package objectrefs.println("Found 20 Novel results"). Listing 5. For this requirement. Non-overwritten searches (four of them) in the PremierSearchTool class will simply be inherited into SuperiorSearchTool by the rule of inheritance.println("Performing enhanced LDAP Search").

Very good.
searchWeb. right? So this is incorrect.searchOracleDB.Chapter 5
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Performing enhanced Novel Search Found 20 Novel results Performing enhanced LDAP Search Found 15 results in LDAP Found 9 results in Libraries
Time for a small quiz. Bob is the creator of search tools and Harry is the user of the search tools. Let’s say we used the following statement to create the search tool in our utility (SearchTools) class. which of the following statement should he be using to create the tool object?
searchWeb.searc hMySqlDB
a) PremierSearchTool ptool = new PremierSearchTool().searchNovel(updated).searchLdap(updated).searchMySqlDB. however the novelsearch and ldapsearch will be the old ones.searchOracle DB. The answers are ‘a’ and ‘b’. Keep it going.searchLdap(old).searchImages. If Harry should only be allowed to use the following 7 search methods. except that he doesn’t need the library search.searchOracleDB.
PremierSearchTool ptool = new PremierSearchTool()
With this he can use all 6 searches. which of the following statement should he be using to create the tool object?
searchWeb. I am sure you knocked out the questions in a sec.
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. Based on the above statement. b) SuperiorSearchTool stool = new SuperiorSearchTool().
This case is same as Case 2. Case 3: Client C requires the following 6 search methods only. 1. searchMySqlDB. answer the following questions. But the requirement says we need the updated ones.searchNovel(updated).searchImages.searchNovel(old). b) SuperiorSearchTool stool = new SuperiorSearchTool().searchImages.searchLibraries (new)
a) PremierSearchTool ptool = new PremierSearchTool().searchLdap(updated). If Harry should only be allowed to use the following 6 search methods. 2.

public class SearchTools{ public static PremierSearchTool getClientASearchTool(){ PremierSearchTool p = new PremierSearchTool(). This is parent = child relationship we read in the previous section. } public static PremierSearchTool getClientCSearchTool(){ PremierSearchTool p = new SuperiorSearchTool (). So.
My utility class will return the search tool to Client C as shown below in bold:
package objectRefs. do we have one? I am sure you guessed the answer. he can also use the new searchLibraries and he should not be allowed to search libraries. right? So.
package objectrefs.5f (ClientCSearchToolTest.5f.
With this he can use all the required 6 searches with updated novel and ldap searches as well. His code will therefore look something like the one shown in listing 5. then it is the right answer. return s.
PremierSearchTool tool = new SuperiorSearchTool(). But you know what.java) Client C search code. } public static SuperiorSearchTool getClientBSearchTool(){ SuperiorSearchTool s = new SuperiorSearchTool (). Listing 5. } } return p. public class ClientCSearchTool {
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. this too is incorrect If we have statement that allows access to all the methods in the PremierSearchTool with only the overwritten methods from the SuperiorSearchTool. return p.
Client C can now use the above class and invoke the method getClientCSearch Tool() to get his search tool. Following is the statement and correct answer.Chapter 5
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Say we use the following statement to create the search tool in our utility (SearchTools) class
SuperiorSearchTool stool = new SuperiorSearchTool().

tool. All methods in parent class with overwritten methods from child class. } }
Compile and execute the test class to see the following result
Found 10 Web results Found 10 Image results Performing enhanced Novel Search Found 20 Novel results Performing enhanced LDAP Search Found 15 results in LDAP Found 2 results in Oracle DB Found 3 results in MySQL DB
When you have parent and a child class you can only get 3 different levels of access to methods in an object. 1. tool. Rule: In Java. provided the class implements that interface.searchOracleDB(). let’s understand one simple rule with objects and interfaces. So what we will do is.searchImages(). // These two methods will be called from child class tool.searchNovel(). you can also assign an object of a class to an interface. All methods in child class ONLY (Case 2) and 3.
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. (Case 3) A good design must provide as many as possible levels of access to methods.
Understanding Interface References
Before we look other cases. tool. tool. tool. implement the same search tool using a better design by using interfaces and see if it provides more levels of access control.Chapter 5
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Understanding Interface References
public static void main(String args[]) { PremierSearchTool tool = SearchTools.searchMySqlDB().searchLdap().getClientCSearchTool(). All methods in parent class ONLY (Case 1) 2.searchWeb().

if we have a class declaration as shown below. public interface InternetSearch { public void searchWeb().B.java)
package objectrefs.B.6b (DirectorySearch.
public class X implements A.C. C c = new X(). let’s distribute these 7 methods into four interfaces namely InternetSearch.java)
package objectrefs.6a (InternetSearch.
A a = new X(). }
Listing 5.
Accessing methods using Interfaces
Using the above concept of interfaces references.C
then the following statements are perfectly legal since class X implements interfaces A.6a-d.Chapter 5
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For instance. let’s see if we can improve our PremierSearchTool and SuperiorSearchTool classes.
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.DirectorySearch. Reference a can be used to access only the methods defined in interface A that are implemented in X Reference b can be used to access only the methods defined in interface B that are implemented in X Reference c can be used to access only the methods defined in interface C that are implemented in X Reference variables a. public void searchImages(). B b = new X().c are called as interface references to object of class X.b. Listing 5. DatabaseSearch and LibrarySearch as shown in listing 5. As we know that the total search methods we have are seven.
With the above statements.

and a library search method which happen to be the method in LibrarySearch interface. we will have this class implement these two interfaces. Listing 5.6e (PremierSearchTool. }
Listing 5.6e-f. Now my updated PremierSearchTool and SuperiorSearchTool classes will be as shown in listing 5.Chapter 5
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public interface DirectorySearch { public void searchNovel(). Make sense? This is case 2 requirement. public void searchLdap(). Since it must also have the two updated directory methods (novel and ldap) which happen to be the methods in DirectorySearch interface. }
Listing 5. public interface DatabaseSearch { public void searchOracleDB(). DatabaseSearch { // Methods from InternetSearch
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. public class PremierSearchTool implements InternetSearch. as it needs the four old search methods. public interface LibrarySearch { public void searchLibraries(). public void searchMySqlDB().java) Updated search class. DirectorySearch.java)
package objectrefs.6d (LibrarySearch.
package objectrefs.6c (DatabaseSearch. it will implement all these three interfaces. }
Since our PremierSearchTool should have six methods (case 1) in the InternetSearch. DirectorySearch and DatabaseSearch interfaces.java)
package objectrefs. Make sense? Our SuperiorSearchTool will again be a child class of PremierSearchTool.

Using the above two updated classes. the previous three cases will also work. if there is a piece of code that is used by multiple clients. but also implements two interfaces. Always keep this in mind. In this case. This object has three interfaces (views) as shown. for some reason when you update the code or redesign. it inherits to get the four old search methods.Chapter 5
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The SuperiorSearchTool class not only extends from PremierSearchTool class. DatabaseSearch
The above class declaration should be viewed as shown in the following figure. You’ll get the same results. This is to say that our new design still supports the old clients without breaking them. Note: In real world applications. In Java. You can copy the above two files. make sure that none of the clients functionality is broken. Now let’s see if this design offers some additional cases. An interface reference is basically a restricted view of the underlying object.B. a class can extend from one class.
InternetSearch DirectorySearch DatabaseSearch
searchWeb
searchImages
searchNovel searchLdap(old) (old) PremierSearchTool
searchOracle
searchMySql
Let’s see what meaning the above figure conveys us. Ok.C. and at the same time can implement any number of interfaces. ok. and again run all the three test classes for the clients A. You need to do lot of regression testing when ever you make changes. DirectorySearch. Trust me. Look at the following class declaration:
public class PremierSearchTool implements InternetSearch. PremierSearchTool object has six methods shown in the rectangle. Since InternetSearch interface defines searchWeb() and searchImages()
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.

Now. we add the following method to the SearchTools utility class to return the search tool as shown below:
public static InternetSearch getClientDSearchTool(){ InternetSearch p = new PremierSearchTool ().6g. Listing 5.
Found 10 Web results Found 10 Image results
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. tool. Client D search code will be as shown in listing 5. }
Using the above method.searchImages(). let’s look at three cases shown below: Case 4: Client D must be allowed to access only the following methods:
searchWeb().6g (ClientDSearchToolTest.Chapter 5
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methods.
package objectrefs.java) Client D test class. } }
Executing the above client test code will produce the following results. public class ClientDSearchToolTest { public static void main(String args[]) { // Get the tool.
So. the above two methods are the InternetSearch interface methods. searchImages()
If you noticed carefully. return p. using its reference one can only access these two methods from the underlying object.getClientDSearchTool(). Same is the case with other two interfaces. InternetSearch tool = SearchTools. So.searchWeb(). tool. I can give the search tool for this client by just creating an object of PremierSearchTool and referencing it with InternetSearch interface type as shown below:
InternetSearch tool = new PremierSearchTool().

Chapter 5
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Accessing methods using Interfaces

The beauty is, though we are giving him the object that has all the six methods, he can only invoke two methods. If he tries to invoke other search methods, he get’s a compilation error. Are’nt we restricting access to some methods? The InternetSearch interface reference acts as a restricted point of entry into the search tool. Tomorrow if Client D requests access for another search method, all we need to do is declaring the new method in the InterentSearch interface. That’s it. Isn’t this simple? Case 5: Client E must be allowed to access only the following methods:
searchNovel(old), searchLdap(old)

‘old’ means methods in PremierSearchTool. Follow the same steps as we did for Case 4. Take this as home work. Case 6: Client F must be allowed to access only the following methods:
searchOracle(old), searchLySql(old)

Follow the same steps as we did for Case 4. Take this is home work. Now, look at the following declaration for SuperiorsearchTool class.
public class SuperierSearchTool extends PremierSearchTool implements DirectorySearch,LibrarySearch

The above class declaration can be viewed as shown in the following figure. (See the figure in the next page).

In the above figure, the first four methods are inherited from PremierSearchTool. class.The other three methods are the methods from DirectorySearch and LibrarySearch interfaces. Let’s look at 2 more cases. Case 7: Client G must be allowed to access only the following methods:
searchNovel(updated), searchLdap(updated)

‘updated’ means methods in SuperiorSearchTool. For this case, we need to return the DirectoryInterface reference of Superior SearchTool object as shown below:
DirectoryInterface tool = new SuperiorSearchTool();

You can then write the new method in the SearchTools class and Client G test code following the steps in Case 4. Case 8: Client H must be allowed to access only the following method:
searchLibrary()

This is like Case 7. Take it as home work.

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Here is a simple thumb rule. If a class implements ‘n’ interfaces, then the number of ways you can access methods using interfaces is also ‘n’. Since PremierSearchTool implements three interfaces, it gave us 3 additional cases. Likewise, Superior SearchTool implements two interfaces, so it gave us 2 more cases. The moral of the story is, more the interfaces a class implements, more the access levels you can get. I know you might have cursed me for not giving the answers for Case 4, 5, and 8. So, here are the answers. Case 4:
DirectorySearch tool = new PremierSearchTool();

Case 5:
DatabaseSearch tool = new PremierSearchTool();

Case 8:
LibrarySearch tool = new SuperiorSearchTool();

Using the above, you can add methods in the SearchTools utility class, one for each case and then write the test code for each of the classes. In Java 50% of the method parameters are usually of type interfaces, and 50% are of type abstract parent classes. Just remember the following three important points, and you can be a Java champ. 1. If any method takes Object as a parameter, you can pass an object of any class to that method. As an example, consider a class having a method as shown below:
public class ArrayList{ public void add( Object obj) { // Some Statemens } }

Since we know that Object is the ultimate parent class, the above method can take any object as shown below:
ArrayList list = new ArrayList(); list.add (new Box()); list.add (new Demo());

In the above code, we can pass any class object to the add() method without any

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Summary

compilation errors. This is because both Box and Demo are the child classes of Object class by default. 2. If a method argument is an interface, then we must pass an object of class that implements the interface. As an example, consider a class having a method as shown below
public class Database{ public void setDataSource( DataSource // Some Statemens } } ds) {

Assuming DataSource parameter as an interface, to invoke the method, we need to create a class that implements the DataSource interface as shown below:
public class TestDataSource implements DataSource{ public void getConnection(){ // Some Statements } }

We then pass the object of TestDataSource as the method parameter .
Database d = new Database(); d.setDataSource ( new TestDataSource() );

3. If a method parameter is any class other than Object class, then If the class is not abstract, then we pass an object of same class. If the class is abstract, then we pass an object of its child class. Just remember these three rules. Read them 100 times. You can do it. This concludes the chapter. We learned a lot in this one. I suggest you to read this chapter at least 2 times. Trust me, if you understood the concepts until now, you can take the liberty to even skip the rest of the chapters if you don’t have time. You’ll still be fine. But I recommend you to spend some time with the later chapters to become even more comfortable with Java.

Summary
When a parent class object is assigned to a parent class reference, we can only invoke the methods in the parent class.

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When a child class object is assigned to a child class reference, we can only invoke the methods in the child class. This includes the methods inherited from parent class. When a child class object is assigned to a parent class reference, we can only invoke the non-overwritten methods in the parent class and overwritten methods from the child class. An object of a class can be legally assigned to interface reference only if the class implements the interface. If an object is assigned to an interface reference, we can only invoke the methods defined in the interface, even though the object has more methods.

Interview Questions
No one will ask you any questions from this chapter during interview, since this chapter gives you better design ideas (not concepts) using the concepts from the previous chapter. All right then, catch you in the next chapter.

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Exception Handling

Chapter Goals

Chapter 6
Exception Handling
By the end of this chapter, you will know for sure as to how to handle abnormal conditions which we call as exceptions in Java programs. Exception handling is one of the most important aspects of any Java based application.

Environment Setup
All the programs in this chapter should be stored in the following directory.
C:/JavaTraining/Chapter6/exceptions exceptions is a package in which we will store the programs. To compile the programs, move to the following root directory and get ready. C:/JavaTraining/Chapter6

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Introduction

Introduction
In the last few chapters, we have written several programs and they all worked fine without exhibiting any abnormal behavior, right? They are therefore good programs for behaving well without giving us any trouble. However, in real world applications we come across several abnormal conditions in programs due to which JVM abruptly terminates the program. There is absolutely nothing wrong terminating the program, but it would be really nice if the JVM tells us what the abnormal condition is, due to which it terminated the program. We are kind of asking JVM to be somewhat liberal by being co-operative. We will find out later if the JVM is really co-operative or not. Sometimes, there will be abnormal conditions which are really abnormal that the program cannot recover from. For instance, if the JVM run out of memory space is there anything the program can do to fix the problem? No, right? In such cases, JVM terminating the program is the best solution. However, there will be several situations where the abnormal conditions from JVM point of view are normal from the program point of view, and we don’t want JVM to terminate the program. In order that JVM not to terminate the program, there must be some mutual understanding between JVM and the Java program. The understanding should be like the Java program requesting the JVM, “Hey JVM, I know you always try to terminate me in the event of an abnormal condition. But you know what, why don’t you tell me what the problem is, so that I‘ll take the decision whether or not to terminate myself”. This is exactly what the JVM does. It wraps all the information about the abnormal condition in an exception object, and rubs off its hands by throwing the exception object to the Java program. Now the exception object is in Java program’s court. The program should sincerely catch the exception object, take the appropriate decision and continue with the rest of the program. This is called Exception handling. However, if the program fails to catch the exception object, then the program terminates. Now tell me this. Is the JVM co-operative or not? Good. JVM is really with us and gave us all the co-operation that it can possibly give. So, lets’ use its support and move on. As a programmer, at any cost we don’t want the program to terminate abruptly. For instance, I have a client who is using my program to calculate his taxes. All of sudden the program terminates abruptly and the data is lost. You tell me, what would be his state in that situation. If my program conveyed a meaningful message to him explaining what the problem is, then he will be more than happy, right? This is exactly what we can do if we handle exceptions.

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Handling Exceptions in Java

From here onwards, let’s write programs that can also handle abnormal conditions (exceptions).

Handling Exceptions in Java
To handle exception objects in our Java program, all we need to do is, 1. Identify the harmful statements that cause the abnormal conditions. 2. Enclose the statements in exception handling block. That’s it. Pretty simple, right? Let’s say we completed Step 1. To complete the second step, we first need to know is what an exception handling block is. An exception handling block is formed using the keywords try and catch as shown below: try { // Harmful Statements } catch ( exception object ) { // Exception handling statements } The exception handling block comprises of try block, immediately followed by a catch block. If one of the harmful statements in the try block causes an abnormal condition, JVM with create an exception object with all the information and throws it. The catch block will catch the exception object and executes the statements in its block. Once all the statements are executed in the catch block, the program continues with the rest of the statements following the catch block, resuming the program execution. Let me tell you one thing. You don’t have to scratch your mind trying to understand exceptions. There are simple tricks that I am going to tell you how to handle exceptions. Just follow them and you’ll be good. Also, you really don’t have to spend too much time on this chapter. I’ll keep this chapter as simple and as short as possible, so that we can spend more time learning the important ones.

If you compile and execute the above program, the result will be as shown below:
Exception in thread "main" java.lang.ArithmeticException: / by zero at exceptions.ExceptionTest.main(ExceptionTest.java:11)

Notice the output. There are three important pieces of information. 1. The exception object belongs to a class called java.lang.Arithmetic Exception. 2. Exception message "/by Zero" 3. Point where the exception occurred "exceptions.ExceptionTest.main(ExceptionTest.java:9)" In the above program we did not handle the exception, so the program abruptly terminated at line 11, and therefore we didn't see the message "Hello World" at line 14. In this case, JVM sincerely did its job of throwing the exception object. But the above program failed to catch the exception object and this is why the program terminated. The exception is mainly because we are dividing with 0 as shown below:
int j = 0; int k = i/j;

This is the culprit statement. So the trick is, simply enclose this statement in a try-catch block as shown in listing 6.2.

In the above program, the try block is pretty obvious. The catch block is the one that deserves some attention. If someone throws a ball at us we need to catch the ball and not something else, right? Based on this analogy if the JVM throws an exception object of java.lang.ArithmeticException class, the catch block must catch the same. This is why the catch statement is like as shown below:
catch(java.lang.ArithmeticException ae)

In the above statement, ae is some name that we gave to the exception object. Don’t worry about why we need the name at this point of time. We’ll see that later. For now, just remember that the catch statement must always give a name to the exception object. In the catch block, we can write any statements to handle the exception, and convey a message that exception is handled. This is what we did in this example. If you compile and execute the above updated program, the output will be:
Arithmetic Exception handled Hello World

Once the catch block is executed, JVM will continue with the rest of the program. This is why you see "Hello World" getting displayed. No more program termination.

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The Real Challenge

The Real Challenge
If you look at the previous example, JVM has thrown ArithmeticException. We came to know about this exception only when we ran the program without the try-catch block, right? Then we smartly put the right name in the catch statement and fixed the problem. Without doing the above, how the hell are we supposed to know what type of exception object JVM throws? There can be infinite different types of exceptions JVM can throw. How do I know which exact exception to handle? We’ll surely address this challenge but only after learning the next section. Keep this challenge in mind for now.

Exception Types
In Java, there are tons of built-in exception classes. JVM can only throw exception objects of these classes only. The good thing is that we will neither have learn about all these different exceptions nor store them in our mind. However, we need to know very few important ones that are most commonly thrown by the JVM. Thank God. In Java, all the built-in exception classes are categorized into two types as, 1. Runtime Exceptions 2. Checked Exceptions Runtime Exceptions as the name suggests, will be created and thrown by the JVM, only when the program is executed. With these exceptions, the compiler doesn't know for sure whether an exception will be really thrown or not. Therefore it compiles the program without any errors even if you don't put a try-catch block. When you run the program, that's when an exception may or may not pop up based on the data. If an exception pops up, then the program terminates. This is the price you need to pay with runtime exceptions. 50-50 chances, right? Checked Exceptions on the other hand are created and thrown by Java programs themselves. Because they are thrown by the Java program itself, the compiler knows exactly what possible exceptions particular statements in a program can throw, and checks whether the program handled these exceptions using try-catch block. Failing to handle will not let the program to compile at all. So, a try-catch block is a must for checked exceptions.

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Exception Types

Now that we know the difference between the two, let’s understand five runtime and two checked exceptions. Fair enough, right?

Five important Runtime Exceptions
java.lang.ArrayIndexOutOfBoundsException This is thrown when we try to access an element from an array outside the bounds of its length. java.lang.ClassCastException This is thrown when we improperly cast objects. We’ll see an example later. java.lang.NullPointerException This is thrown when we try to call a method on a null object. Most frequent one. java.lang.ArithmeticException Thrown when illegal arithmetic operations are performed. java.lang.ClassNotFoundException Thrown when JVM is unable to load a class.

Two important Checked Exceptions
java.io.IOException Thrown when working with files. java.io.FileNotFoundException Thrown when a file is not found. In Java, all the built-in exceptions are nothing but classes. We really don’t have to worry about how the above classes are written and all that. All we need to know is how they are related with each other. To understand this, look at the following inheritance diagram.

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Object

Throwable Error Exception

RuntimeException IOException InterruptedException Checked Exceptions NullPointerExcepti on and many more

ArrayIndexOutOfB oundsException Runtime Exceptions

ArithmeticExcepti on

and many more

Fig 6.1 Exception hieracrchy

Look at the above hierarchy. As I said before, Object is the ultimate parent of all.The actual exception class hierarchy begins from the Throwable class. This is the ultimate exception parent class. This class has two child classes namely, Exception and Error. In Java, we must never handle errors. Good. This leaves us with just one option, which is handling Exception objects (ones in the rectangle). Therefore, we are only interested in the hierarchy enclosed in the rectangle. If you notice the hierarchy in the rectangle, 1. All the Checked Exceptions (within upper dotted line) are child classes of Exception class. 2. All the runtime exceptions (within lower dotted line) are child classes of RuntimeException class which in turn is the child class of Exception class. It is the Exception class at the top that will help us to address the challenge we faced before. With this knowledge let’s get back to writing programs. Take a look at the code in listing 6.3.

When you compile and execute the above test program, it will produce an Arithmetic Exception due the following statement:
int k = i/0;

So, all the following statements will be skipped, and the control comes to catch block that handles ArithmeticException. So, the output of the program will be,
Arithmetic exception handled DONE

Knowing the problem, let’s fix it by updating the division statement to
int k = i/2;

Good. Compile and execute the program only to see another exception as shown below:
Hello 1 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2 at exceptions.ExceptionTest1.main(ExceptionTest.java:16)

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java) Multiple catch blocks.out.println("Hello 2"). how should we make the program to also handle ArrayIndexOutOfBoundsException? Very simple.out. Aren’t we going out of bounds? So is the exception. This is the first of the five runtime exceptions we listed before.Chapter 6
Exception Handling
Exception Types
Now we started to hate exceptions.println("Out of Bounds handled").println("Hello 1"). a[2] = 3. // This is the culprit System.4 (ExceptionTest4. but trying to store the element in the 3rd position that doesn’t even exist. the statement shown below is the culprit.4. the control goes to the catch block that handles it (second one). The question is.out. // We fixed this System. a[0] = 1. } }
This time when the out of bounds exception is thrown.
a[2] = 3. public class ExceptionTest4 { public static void main(String args[]) { try { int i = 10. Listing 6.
package exceptions. a[1] = 2.println("Arithmetic exception handled"). Since the exception message gave us the line number as 13. We fixed one. Put another catch block for this exception as shown in listing 6.out.
If you look at the code. the program continues with the statements after the last catch block. Shoot! This time my new exception is ArrayIndexOutOfBoundsException. } System. The output of the above program will therefore be:
Hello 1 Out of Bounds handled DONE
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. } catch (ArrayIndexOutOfBoundsException ae) { System.out. int k = i / 2. } catch (ArithmeticException ae) { System. int a[] = new int[2]. we created an array of size 2. and we got another.println("DONE"). Upon executing this new catch block.

// Second culprit System.out. The time the result will be. a[1] = 2.println("DONE"). we activated both the culprits. int k = i / 0. Look at the code in listing 6.Chapter 6
Exception Handling
Exception Types
So the moral of the story is.5 (ExceptionTest5. } }
In the above code. a[0] = 1. int a[] = new int[2]. a[2] = 3.
Hello 1 Exception handled DONE
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.println("Hello 1"). } catch (Exception e) { System.out.5. and the output will be:
Exception handled DONE
Now.println("Exception handled"). Is there a cleaner solution that addresses all the above problems? Yes.out. a single try block can be associated with ‘n’ number of catch blocks each catching its own exception. // First Culprit System. You know what. we should still know the exact names of all 100 exceptions. public class ExceptionTest5 { public static void main(String args[]) { try { int i = 10.println("Hello 2").out.
package exceptions. all our code will be cluttered with catch blocks than the real code. I am stuck. and placed 100 catch blocks. } System. Now we are smart. we have. Holy cow! Now. This time after the Hello 1 statement.java) Single generic catch block. Listing 6. the out of bounds exception will be thrown. it first throws ArithmeticException. What if my try block throws 100 different exceptions? Should we put 100 different catch blocks? Even if we are patient enough to add 100 catch blocks. Some how we found the names of 100 exceptions. When you run the program. let’s just fix the first culprit by making it i/2.

This is like a consistent catch. D. the catch block will still catch it. Some piece of code throws five different exceptions namely A.E. right?
catch (Exception e)
All this happened because Exception is the parent class of all the exceptions. strictly speaking if we use this type of catch statement. it also suffers from a serious drawback. exceptions A.Chapter 6
Exception Handling
Exception Types
Wow. This single catch block handles all the exceptions in the same way.B. Hurray! Just remember this one catch block an you can handle any exceptions. It’s like a super cop catching all the culprits. Now you can relax and read the next topics without any worries. For instance. You will see the same ‘Exception Handled’ message for both ArithmeticException as well as ArrrayIndexOutOfBoundsException. Sorry guys. This generic catch block is indeed the solution for the challenge we had couple pages ago. If it is ok for you to handle all the exceptions in the same way. So. The single magical catch block shown below caught both the exceptions unlike before. The requirement is that. then use the single catch solution trick. It all boils down to how you want to handle exceptions. Haha. which is using multiple catch blocks. Otherwise. How do you implement this scenario? Look at the following two solutions:
public class ExceptionTest { public static void main(String args[]){ try{ // Statement throws exception A // Statement throws exception B // Statement throws exception C // Statement throws exception D public class ExceptionTest { public static void main(String args[]){ try{ // Statement throws exception A // Statement throws exception B // Statement throws exception C // Statement throws exception D
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. we don't have to know about any of the infinite exceptions we have. Damn. If the try block throws some other exception. What a catch block. while the remaining exceptions C. Some times being consistent is bad. The trick doesn’t work all the times. consider this scenario. E can all be handled in the same way. Though the above trick is useful. use the multiple catch solution. Can someone please answer the following questions? How do you know which exception really popped up? What if I want to handle different exceptions differently? The answer to both the questions is the same. Life is cool :-) I don't have to learn anymore in exceptions. B must be handled differently.D. The reason is simple.C. Look at the figure couple pages back to verify this.

} catch(Exception e ) { System. d. then it will also swallow exceptions A and B.out.} catch(C c){ System.getTime().out. and we are trying to invoke a method on it. Put a separate catch block for every exception you need to handle differently. if you have multiple catch blocks. and so is the NullPointerException. you put the generic (trick) catch statement for all the remaining exceptions that you want to handle in the same way. If we try to put it as the first catch block. when an object is not created. In Java. we simply have to check that the object is not null before invoking the method as shown below:
Date d. in the above code. Compiler recognizes this fact upfront and gives you an error saying “Unreachable catch blocks”. the A and B catch blocks will never be reached by the JVM to execute. Now.} catch(D d){ System.
In the above statements. Null object means the object doesn’t exist at all.out. if ( d != null){ d.} catch(B b){ System. the object pointer ‘d’ is null.} }
Which one of the above two implementations do you like? I like the second one. it’s time to understand the most common exception that any Java program faces. }
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.println("Handled B"). So.println("Handled"). It’s the very popular NullPointerException. the generic catch block must be the last catch block. right? So. right? Look at the following statements. The rule is simple. This exception is thrown when we try to invoke a method on a null object.println("Handled B"). the default value of any object reference variable is null.getTime().} catch(E e){ System.out. To avoid this exception.println("Handled A"). the rule is always to put the parent class exception after the child class exceptions.println("Handled").println("Handled").println("Handled A"). Very important rule.out.Chapter 6
Exception Handling
Exception Types
}
// Statement throws exception E } catch(A){ System.
Date d.out.out.} }
}
// Statement throws exception E } catch(A a){ System. When we don’t create an object using new keyword.out.} catch(B b){ System. the Date object is actually not created. Therefore.println("Handled"). Can you tell me when an object doesn’t exist? The answer is very simple. Lastly. So.

Let’s take the following built-in checked exception class. I know you are curious to know.
java. To convey an error message we need do the following things: 1. Throw the exception object and 3.
Handling Checked Exceptions
Handling checked exceptions is no different from handling runtime exceptions. Let’s look at both the cases.io is the package in which IOException class is stored in the library) To create an exception of the above class. why the hell the program should create and throw exceptions. This is a good practice. that exceptions are both good and bad. Handle the exception object using try-catch block and display the message. Case 1: Creating an exception from built-in class. These exception classes can either be built-in classes or user defined custom exception classes. In real world applications. right? I told you at the very beginning of the chapter. it’s extremely important to convey meaningful error messages to the users. the Java program itself will create. JVM creates and throws them. convey proper error messages. With runtime exceptions as I said before. We saw the bad side of exceptions until now.io. creating object of one such class.IOException
(Note that java.Chapter 6
Exception Handling
Handling Checked Exceptions
The above code will never throw a NullPointerException because of the check we are doing. and why should it handle them again? Doesn’t make sense. Creating an exception means.
Creating Exceptions
All the exceptions in Java are nothing but classes. We just have to handle them using try-catch block. Let’s now look at the good side of exceptions. However with checked exceptions. This is exactly what we can do with exceptions. throw and handle exceptions. We still use the same try-catch blocks. Create a checked exception object with a meaningful message 2. Always check for the object reference variable as not null before invoking the methods using it. we simply create an object like we did using new keyword as shown below:
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.

In this case. (or) java. } }
If you look back at our exception class hierarchy diagram.Chapter 6
Exception Handling
Handling Checked Exceptions
String message = "We are unable to locate the file.
You can define a message and pass the message to the exception object or can directly pass in the message as shown above. Please try again later". we need to first write our custom exception class as shown below:
public class MyCustomException extends Exception{ public MyCustomException ( String message ){ // Pass the message to the parent class constructor super ( message ). This is how we create exception object ex from built-in exception class. Easy. Therefore.IOException ex = new java.
If someone asks you to create a custom exception. java.io. Most folks use the second approach.IOException( message ). Keep this in mind since we often create whole lot of custom exception classes in real world applications. a) We must create a class that extends from Exception class as shown above. b) We then have to define a constructor that takes String as an argument for message and then pass this message to the parent class Exception constructor using super statement as shown above. Finally we create the exception object as shown below:
MyCustomException ex = new MyCustomException ("Please call 1-800-8888244 for Help"). this is what you need to do. We now created our own exception class.io. to create a custom checked exception.io.IOException ex = new java.
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. right? Case 2: Creating an exception from a user-defined or custom exception class.Please try gain later"). That’s it.io. all the checked exceptions are child classes of Exception class.IOException("We are unable to locate the file.

See listing 6. The second one is simplest as it does all in one line. public class MyCustomException extends Exception { public MyCustomException(String message) { // Pass the message to the parent class constructor super(message).
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.java) A custom exception class
package exceptions.6. throwing and handling should be done in the same method in the Java program (at least for now). To throw an exception we simply use the throw keyword as shown below:
MyCustomException ex = new MyCustomException (" Please call 1-800-8888244 for Help "). This is the commonly used convention. Knowing all the above three things like creating.Chapter 6
Exception Handling
Handling Checked Exceptions
Throwing Exceptions
Now that we know how to create checked exceptions. we need to see how to throw these exceptions. Like we did with runtime exceptions. Keep it in mind.
Use can use one of the above conventions.6a (MyCustomException. throw ex.
Handling Custom/built-in Checked Exceptions
This is the simplest of all. we also use the same try-catch block for handling the custom/built-in checked exceptions as shown below:
try { // Statements to create and throw exceptions } catch ( MyCustomException ex){ // Do what ever you want here }
One important point to be noted here is that. all the above 3 steps namely creating. (or) throw new MyCustomException (" Please call 1-800-888-8244 for Help "). Listing 6. throwing and handling checked exceptions. let’s write a program to get a complete picture.

out.Chapter 6
Exception Handling
Handling Checked Exceptions
} }
Listing 6. let’s fix the code as shown below:
if (j == 0) { try{ throw new MyCustomException(“Call 1-800-888-8244 for Help”). System.out. This method creates and throws the exception object if the value of j is 0. it computes and displays the result. } } }
If you look at the above two classes in listing 6.println (message). We then wrote a class named Calculator which defines the divide() method.java) Method throwing an exception
package exceptions. The statements in the catch block. } }
Compile the code again with the above fix.
The above compilation error is pretty obvious because the method missed the 3rd step.java Error: Unhandled exception type MyCustomException. Try compiling the above classes as shown below:
C:>JavaTraining>Chapter6>javac exceptions\*.6a-b. } catch (MyCustomException ex){ String message = ex. which is handling the exception. } else { System. int j) { if (j == 0) { throw new MyCustomException("Call 1-800-888-8244 for Help").println(i + "/" + j + "=" + i / j). public class Calculator { void divide(int i.getMessage(). right? Where the hell did the
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. and the error disappears. we first created our custom checked exception class which we already know. right? So.6b (Calculator. Otherwise. I know what you are wondering about.

If the method doesn’t know how to handle a checked exception.
Since it returns the message. Listing 6. but should not handle the exception at all. look at the test class shown in listing 6. Is this possible? Don’t we get a compilation error when we do this? You know what. 2). we inherited it from Exception class. the first call to divide method produces the correct result without any exception. there is a solution. This is possible only by throwing the exception and then handling it. all of the methods in the built-in Exception class will be inherited into our MyCustomException child class. Say we have a requirement where the above divide() method can create and throw an exception. it must throw the exception to the
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. 0). Now.java) Test class for exceptions. So. One such method is the getMessage() method shown below: getMessage() – This method returns back the String message that we supplied while creating our exception as shown below:
MyCustomException("Call 1-800-888-8244 for Help "). calc. It is the second one that results in an exception which the divide() method handles and displays the message.divide(10. Java is all about give and take. like good folks we took it into the message variable. by the rule of inheritance. public class ExceptionTest6 { public static void main(String args[]) { Calculator calc = new Calculator(). right? Let’s introduce a tricky situation here. // Bad one } }
In the above test class. Compile and execute the above program to see the following result. // Good one calc.
package exceptions.Chapter 6
Exception Handling
Handling Checked Exceptions
getMessage() method come from? If you look at our custom exception class.6c.
10/2 = 5 Call 1-800-888-8244 for Help
The above error message is more meaningful and my client will be happier then ever. we simply echoed it.6c (ExceptionTest6. Once we got the message.divide(10.

Again.
package exceptions. See listing 6. The syntax for throwing an exception to the outside world is shown below: void somemethod() throws <list of exceptions> So let’s update our divide() method by removing the try-catch block and using the throws keyword to throw the exception to the invoker of the method. } } }
As you can see from the above code. See listing 6.6e (ExceptionTest7.out.println(i + "/" + j + "=" + i / j). public class Calculator { void divide(int i. } else { System. This is like the method saying. Smart method. my code might throw an exception named MyCustomException which I honestly don’t know how to handle it.6e.java) Updated calculator class using throws keyword. You guys take are of it. This is how it rubs off her hands. Then the one who actually invoked the method must handle the exception. Bye”.
package exceptions. it should throw it to some other method.Chapter 6
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Handling Checked Exceptions
outside world using the throws keyword.6d (Calculator. public class ExceptionTest7 { public static void main(String args[]) {
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. right? Now the one who invokes the method must handle this exception by putting the try-catch block around it.java) Caller handling the exception. Listing 6. “Hey guys. the divide() removed the try-catch block. This is like throwing the ball until some one finally catches it.6d. if the called method doesn’t handle the exception. This poor guy will be cursing the divide() method and painfully adds a try-catch block. and declared a throws clause specifying the name of the exception to throw to the caller of that method. int j) throws MyCustomException { if (j == 0) { throw new MyCustomException("Call 1-800-888-8244 for Help"). Listing 6.

We call this as exception stack trace.println(message). } } }
The try-catch block is now moved into the test class and all the calls to the divide() method must be enclosed in this block.divide(10. Though this looks weird at the first place.java:8) Exception in thread "main"
If no one handles the exception. It tells you the following things:
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. Modify the test class by removing the try-catch block and declare a throws clause in the main method as shown below:
public static void main( String args[]) throws MyCustomException { Calculator calc = new Calculator(). it can. try { calc. // Bad one } catch (MyCustomException ex) { String message = ex. if we observe it carefully.2). The result will still be the same.main(ExceptionTest. but you know what.divide(10. JVM will then display a weird exception trace on the console. we can’t wait to appreciate what it has to offer.divide(10. the output will be
10/2=5 MyCustomException: Call 1-800-888-8244 for Help at Calculator. 2). // Good one calc.0). calc. Let me ask you a question. With this updated code for both Calculator and ExceptionTest7. 0). this is what JVM will give us.getMessage().out.divide(Calculator.divide(10.java:9) at ExceptionTest. // Bad one }
If you execute the test class with the above update. What if the test class too doesn’t want to handle the exception? Can it also throw the exception to some other world? Yes.Chapter 6
Exception Handling
Handling Checked Exceptions
Calculator calc = new Calculator(). that world is the final world which is none other than JVM (because the main method is called by JVM). compile and execute the test class. System. // Good one calc.

Let me ask you a question here. A method can throw more than one exception. You agree with me? Note: Whenever you see an exception. Let’s see what this is all about.compute().Chapter 6
Exception Handling
Finally block
1. } catch(IOException e1){ } catch(TestException1 t1){ } catch(TestException2 t2){ } (or) try{ calc. What should I do? The answer
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. TestException 2…. one for each (or) one catch block with the generic Exception as shown below:
try{ calc. All we need to do is. 3. and then moved to line 8 in the main() method. In this case. What ever be the case. What is the exception message like Call 1-880 etc. Can a single method throw more than one exception? The answer is yes. it says it first originated in the divide() method at line 9. using finally block. The way the exception propagated. if you start reading.compute(). TestException1. must handle all the exceptions by putting multiple catch blocks. I always intend to execute some functionality. What exact exception it is. I am sure this information is more than enough to help us fixing the problem.
Finally block
Let's say we have some piece of code that may or may not throw an exception. } catch( Exception e){ }
We finally arrived at the last concept of this chapter. list all the exceptions in the throws clause with a comma delimiter as shown below:
public void compute() throws IOException. Take it positively and be thankful for JVM for giving all the information about the error. in this case it is MyCustomException.. don’t panic. 2.
Then whoever calls this method.

public class FinallyDemoTest { public static void main(String args[]) { Calculator calc = new Calculator().7 (FinallyBlockDemo. 2).Chapter 6
Exception Handling
Finally block
is use finally block and put all the functionality there.divide(10. 1. To get a clear picture.java) Class using finally block.println(message).
10/2 = 5.out. 10/5 = 2. 5).
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.7 Listing 6. The moral of the story is. The result will be. } finally { System.getMessage().println("This is always executed"). Exception handled This is always executed.divide(10.
Change the second compute method to divide(10. There can be only one finally block per try-catch block. } } }
If you execute the above code. System. both the divide() methods are passing good data. and the result will be
10/2 = 5. calc. It must be placed after the last catch block of the try block. 2. } catch (MyCustomException ex) { String message = ex.
There are couple of rules here with finally block. try { calc. This is always executed. We usually place the clean up statements here. the statements in the finally block will always be executed whether or not an exception is thrown.0). and neither an exception will be thrown nor will be handled.out.
package exceptions. take a look at the code in listing 6.

println(“This is never executed”). Exceptions are classified as Runtime Exceptions and Checked Exceptions.exit(0) in both try and catch blocks. To prevent the termination of the program. it’s always nice to know something about exceptions and their details. exception handling is one of the important aspects. simply enclose that method call in a try-catch block. Here is a simple trick with exceptions. This is all about exceptions you need to know.exit(0). }
Interview Question: How can you make the program not execute the finally block? Answer: By placing a System. Trust me. } catch( Exception e){ System. I am sure you thoroughly enjoyed this chapter like others. This will save your life for sure. Before we get there. So. } finally{ System. there is one way we can stop the JVM from executing the finally block. That’s it guys.Chapter 6
Exception Handling
Summary
However. The catch block catches the exceptions thrown by the try block and takes the appropriate action.
Summary
Exception is an abnormal condition that occurs in a program due to which JVM terminates the program abruptly. Trick: If any method in a class has a throws clause in its declaration.out. life get’s real easy and easy from next chapter onwards. This is by manually terminating the program in the try block or the catch block as shown below:
try{ System. The try block encloses all the harmful statements that result in throwing exceptions. In real world applications. In Java. exceptions must be handled. let’s summarize this chapter. exceptions are handled using try-catch block.
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.exit(0).

it must throw the exception to the invoker of that method using throws keyword. The invoker of the method must then handle the exception using the try-catch block. The program must always handle these exceptions using try-catch block. Which of the following is a Runtime exception
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. Which of the following exceptions need not be handled. we need to create a child class of Exception class and define a constructor that takes String argument. A method can also throw multiple exceptions.
Time to play 50-50
1. All the checked exceptions are child classes of which of the following exception class. a) RuntimeException b) Exception 4. the method must declare all the exceptions in the throws clause. If the method decides not to handle the exception. Use finally blocks to always execute some statements irrespective of whether an exception is thrown or not. Checked Exceptions on the other hand are created and thrown by the program itself. If a method throws an exception using throw keyword.Chapter 6
Exception Handling
Time to play 50-50
Runtime Exceptions are created and thrown by the JVM when the program is executed. and yet the program compiles without any errors a) Checked Exceptions b) Runtime Exceptions 3. it must also handle the exception using try-catch block. The parent class of all the checked exceptions is Exception class. A method can throw an exception using throw keyword. In this case. To create a custom checked exception. the program will compile without any errors even without a try-catch block. Which of the following keywords are used for handling exception? a) try-finally b) try-catch 2. Failing to handle will result in compilation errors. With these exceptions.

out. To create a custom checked exception class. which of the following classes it must extend from: a) RuntimeException b) Exception 7.Chapter 6
Exception Handling
Interview Questions
a) NullPointerException b) IOException 5. A NullPointerException is caused by which of the following: a) Invoking a method after creating an Object b) Invoking a method without creating an Object 6.exit (0).println(“Program terminated”). b) System. Which of the following is used prevent the execution of finally block? a) System. Which of the following must be used to always execute the statements whether or not an exception is thrown? a) catch blocks b) finally blocks 10.
Interview Questions
Question: What is an exception in Java?
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. Which of the following keyword is used in the method declaration to throw the exceptions to the outside world? a) throw b) throws 9. Which of the following keyword is used by a method to throw an exception? a) throws b) throw 8.

the program will not compile without a try-catch block. and what is the difference? Answer: Runtime exceptions and checked exceptions are the two categories. Question: What is the only way to bypass a finally block? Answer: Use System.exit(0) statement in try and catch block. Question: What are the two categories of exceptions. Let’s move ahead. This concludes the chapter. The difference between the two is that. Question: What should you do to handle exceptions? Answer: Use try-catch blocks. with runtime exceptions.Chapter 6
Exception Handling
Interview Questions
Answer: An exception in Java is an abnormal condition due to which JVM terminated the program abruptly. the program compiles without any errors even without try-catch block. Question: What are the keywords by which a method can throw an exception? Answer: throw and throws.
That’s it guys. With checked exceptions.
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. Question: How do you create a custom exception? Answer: To create a custom checked exception we need to create a class that extends Exception class and define a constructor that takes a String argument.

Chapter 7
Core Libraries and Best Practices
Chapter Goals
Chapter 7
Core Libraries and Best Practices
By the end of this chapter. Understanding this chapter gives you an edge for a successful career in Java.
C:/JavaTraining/Chapter7
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. Java is all about writing simple programs using the built-in classes. To compile the programs.
Environment Setup
All the programs in this chapter should be stored in the following directory. you’ll be an expert in using Java built-in core classes. This chapter will also tell you some simple tricks and tips on how to use classes and also demonstrates the coding conventions and best practices that are well and truly followed in all the real world Java applications.
C:/JavaTraining/Chapter7/utilities utilities is a package in which we will store the programs.
Chapter Goals
Understand the important core libraries Understanding the most important core library classes Coding conventions and best practices Generating Java Documentation using javadoc tool.
move to the following root directory and get ready.

and leaving the rest to us. By making our Java program use the ready made components. it would be more beneficial if there are some ready made components that can do the bulk of the job. handling abnormal situations in a program using exceptions and may more. The best part of it is. right? 99% of the any real world applications just use these two packages. This is why I like Java. ok. not only the development gets faster. should we learn all of them? Don’t worry. Using the built-in classes is a fine art and mastering this art is the key to become a successful Java programmer. The two packages are listed below: java. Since time is the primary constraint. these components are nothing but the classes.
Important Core Libraries
In Java. So. I am going to tell you few tricks and tips. Trust me. we are only interested in just 2 packages. All the examples we wrote thus far didn’t use any of the built in classes that Java comes with. we just have to understand few classes in each package. A package as we already know is nothing but a collection of classes.
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. mastering this art just takes few minutes. Let me tell you one thing here. In this chapter we'll first understand the important built-in classes and then start writing programs using them. In Java. Let’s don’t waste any time and understand these few classes to become a master Java programmer. We’ll follow them and make our life easy. Fair enough. but also the code becomes more reliable since the ready made components are built by reliable people. In real world applications there will be several complex requirements and it’s time consuming to implement each and every requirement from scratch.Chapter 7
Core Libraries and Best Practices
Introduction
Introduction
In all the previous chapters we have written several programs understanding the object oriented concepts.util and
The above two packages have tons of interfaces and classes. all the built-in classes are very cleanly organized in the form of packages. Again. Awesome. Though there are infinite packages in the Java core library.lang java.

we are interested in just the following classes: 1.
Wrapper Classes
In Java there are some special built-in classes that can only work with objects and not primitive variables like int. there is no way we can use the so called special classes. if we have primitives in our hand.9% of the time. using the methods in the class. Note the class names start with uppercase letter. right? If I cannot move forward without using the special classes. and vice-versa. a Java class will simply have no more than variables and methods. Wrapper Classes 2. let me tell you this one more time. long. So. Table 7. StringBuffer class Before using the above classes. String class 3. Using a class means. which can then be used with the special classes. 99. Wrapper classes simply convert the primitives into equivalent objects. All we need to know is how to wrap a primitive variable and convert it to object.1. There is one wrapper class for every primitive data type as shown in Table 7.Chapter 7
Core Libraries and Best Practices
java. then I definitely need someone to convert the primitives to objects.
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.1 Wrapper Classes Primitive int short byte long float double char Equivalent Wrapper Integer Short Byte Long Float Double Character
The wrapper class names are pretty obvious. That someone is none other than wrapper classes. Don’t worry about the other 0. double etc.1% special classes for now.lang package
This package though contains several classes and interfaces.lang package
java. So without wasting any time. let’s see how to do this.

Given a number as String. we use
int val = ii. Integer ii = new Integer( i ). to convert to primitive int we use the following:
String s = "10 ".
ii is now an object that contains 10. Usage: To convert a double to an object we do it as shown below:
double d=12. To get the primitive back from the object.
Given a number as String.2345.
dd is now an object that contains 12. Usage: To convert an int to an object we do it as shown below:
int i=10.parseDouble( s ). To get the primitive double back from the object.
Double This class is an object representation of double primitive variable. int val = Integer. Double dd = new Double( d ). to convert to primitive double we use the following
String s = "12.intValue().lang package
Integer This class is an object representation of int primitive variable.2345 ". we use
double val = dd.doubleValue().Chapter 7
Core Libraries and Best Practices
java. double val = Double.parseInt( s ).
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.2345.

} }
In the above code. If at anytime we use a built-in class. This is all you need to learn about the wrapper classes.1. it’s time to understand the most important and widely used class. we need to convert it to Integer object and then pass it as a parameter. the usage is same for the rest of primitives.Integer. // Pass the object ii as a parameter add(ii).
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.println(val). So. Let’s do one quick example. Finally. the String class. if we have a primitive int.lang package
Likewise.lang package. look at the import statement that we added at the beginning of the program. } static void add(Integer object) { int val = object. we have a method named add() whose parameter is of type Integer.intValue() + 5.java) Using wrapper classes. public class WrapperDemo { public static void main(String args[]) { int i = 10.
Compile and execute the above program to see the result 15 displayed.lang. Since Integer class is in java. Listing 7.lang. System. we need to import the class as shown below. The add() method takes the Integer argument. computes the sum by converting back to the primitive. import java.1 (WrapperClassDemo.
package utilities. the import statement will be.out. See the code in listing 7.Integer.
import java.Chapter 7
Core Libraries and Best Practices
java. Now. This is exactly what we did in the main method. // Wrap i in the object Integer ii = new Integer(i).

it offers several methods to process the text. state. You agree with me? I am sure you might be wondering that we used strings to store text data in previous chapters. doesn’t this shortcut representation look like a primitive data type declaration? Yes. Ok. there will be million places to store text. it’s really tedious to always use the new keyword and then pass the text.Chapter 7
Core Libraries and Best Practices
java. I said that string is like a special data type and now is the time to understand why it is indeed special.
Now the above declaration is simple. This is because most of the real world data is represented in the form of text. This shortcut is only applicable to String class. So why are we discussing this again? You are right.
All the text must be enclosed in the double quotes. country and what not. Every character in it will have a position or index. and pass the text as shown below:
String str = new String(" This is a simple text message ").lang package
Handling Strings
String class is the most widely used library class. So. city. In any real world application. ok. For all the following examples. we need to create an object of String class. To store text data. and gave us a shortcut representation for strings as shown below:
String str = "This is a simple text message ". So. it does look like that. Let’s see the most important methods. no more using new keyword when it comes to declaring strings. Everything can be represented as text. right? Is there a simpler solution? Fortunately. address. Consider the following string. By looking at it. The indexing of characters will start with 0 as shown below:
0 T 1 2 h i 3 4 s 5 i 6 s 7 8 r 9 e 1 0 a 1 1 l 1 2 l 1 3 y 1 4 1 5 r 1 6 e 1 7 a 1 8 l 1 9 l 2 0 y 2 1 2 2 b 2 3 i 2 4 g 2 5 2 6 t 2 7 e 2 8 x 2 9 t
Let’s look at the methods in String class for working with text data. There will hardly be any Java program without using this class. This is why I called it as special data type for storing text data. Let me say one thing here. But you know what. please use the above table. the inventors of Java language recognized this fact upfront. Once the text is declared using String class.
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. For instance things like customer name.

charAt( 6). Example:
String str = "This is really really big text ". Example:
String str = "This is really really big text ".out.println("The length of the text is " +
length ).indexOf("realy"). int pos2 = str.
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.Chapter 7
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java. int indexOf(char ch) int indexOf(String s) There are two overloaded indexOf() methods.indexOf( ‘s’).lang package
1. int length() : This method returns the length of the text. The first one returns the text from the specified index to the end of the text. int end) The above two overloaded methods return part of the text from the original text.
pos1 will be 3. int length = str.
ch will now have the character ‘s’. One takes a single character and returns the position of its first occurrence. System. and pos2 will be 8. String substring(int start) String substring(int start. char ch = str. and the second takes a string. char charAt(int index): This method returns the character at the specified index. Example:
String str = "This is really really big text ". int pos1 = str. 3. 4. and returns the first occurrence of that string.
The above statement displays the following:
The length of the text is 30
2. The second one returns the text between two specified indices.length().

println(" Strings are equal").println("Strings are unequal"). Example:
String str = "Hello". }else{ System. String s2 = str. boolean equalsIgnoreCase(String anotherString) This method compares the text with another text.equals ("hello") ){ System. The comparison is case-insensitive. and the content in s2 will be really really. The comparison is case-sensitive.
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. 6..out. and returns either true or false. String toUppercase(): This method returns the text in uppercase. if ( str. you can call the method from conditional statements like if.println("Strings are unequal"). if ( str.lang package
Example:
String str = "This is really really big text". This is because ‘h’ is in lower-case.println(" Strings are equal"). }
The above code displays Strings are unequal.out. Tip: If any method returns a boolean. }else{ System. }
The above code displays Strings are equal.
The content in s1 will be big text. Example:
String str = "Hello". boolean equals(Object anotherString) This method compares one text with another text.substring(22). String s1 = str.while etc.Chapter 7
Core Libraries and Best Practices
java. 7.equalsIgnoreCase ("hello") ){ System. 5.substring(8.out. and returns either true or false.out.20).

For complex text processing always try to use StringBuffer class instead of String class.substring(3.
The length of the text is 30 s 6 8 big text really reall Not Equal. Equal
Note: The substring method will throw StringIndexOutOfBoundsException when negative numbers are passed as shown below:
str. This is because string objects are immutable. Following is how we use this class. It is important to note that this way creating of String objects is done purposefully to improve the performance. More processing means more string objects in memory. When we already have the String class that does this. Check the case.
StringBuffer
The StringBuffer class is another class used for processing the texts. The answer is simple.Chapter 7
Core Libraries and Best Practices
java.
To add text. This means. But many a times this solution becomes an anti-performance while complex text processing is involved. we use the append() method as shown below:
buffer.append("A silly young cricket "). -1).lang package
} } }
Compile and execute the above class to see the following result. String class consumes more memory when processing texts than StringBuffer class. With StringBuffer class. whenever we try to change the text in the string object.
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. In such situations. JVM will create a new object. why do we need another class? Great question. there will just be one object in the memory and all the text processing is done within the same object. Create the StringBuffer object as shown below:
StringBuffer buffer = new StringBuffer(). StringBuffer class is an ideal candidate.

See listing 7. buffer. System. String finalText = buffer. StringBuffer) we need to know in the java.
The above will insert Hello after the 5th character.lang package
buffer.append(" through the warm sunny days "). Finally. Listing 7. buffer.append(" through the warm sunny days").out.insert(5.java) Using string buffer class. and you’ll learn automatically when you start writing programs.toString(). There are several other methods in above
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. } }
Compile and execute the above program to see the following result:
A sillHelloy young cricket accustomed to sing through the warn sunny days
This is all about processing text using String and StringBuffer classes.
See how easy it is to process text using this class.append(" accustomed to sing").println(finalText).toString(). buffer.3 (StringBufferDemo.lang package.
package utilities. "Hello").Chapter 7
Core Libraries and Best Practices
java.
You can insert a text in the middle using the insert() method as shown below:
buffer. buffer. we use the following statement:
String finalText = buffer. No big deal.3. Let’s write an example.append(" accustomed to sing ").append("A silly young cricket ").insert(5. to get the text out of the buffer. These are very simple to use. Most of the real world applications use these classes extensively and understanding them will make our life easy. public class StringBufferDemo { public static void main(String args[]) { StringBuffer buffer = new StringBuffer(). "Hello "). These are the only two classes (String. buffer.

Instead we store the data in the form of object. deleting etc. Therefore we need some thing that provide flexible way of storing and retrieving data objects. we will learn only those that are frequently used in real world applications. The classes in this package are called as collection classes. One dimensional data structures 2.util package.Chapter 7
Core Libraries and Best Practices
java. most of the data is highly complex in nature.
java. The next package we are going to see is the java. is a package that has several utility classes that we can use in our programs. This is where the data structures in this package come in handy. but also provide several convenient methods to process the data like sorting. The collection classes in this package can only store objects. let’s find out why we need to use these classes. Before we learn the usage of the classes in this package.util Package
three classes. They cannot store primitive data types like int. In any real world applications. There are two broad classifications for all the data structures in this package as shown below: 1. and it’s extremely tedious to store the data using primitive variables like int. All the data structure classes in this package forms the collection hierarchy.double etc. Two dimensional data structures
One dimensional data structures
The class and interface hierarchy for one dimensional data structures is shown in the following figure. These classes represent various data structures that can be used for storing complex data. double etc. Not only the classes in this package allow storing and retrieving data objects. Point to be noted. and you can try them to see if they help you in any other way. Though there are several of these classes.
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.util Package
This package as the name suggests.

Set and List are the child interfaces of Collection interface.
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. The classes that implement this interface are ArrayList and Vector. Though each class and interface has several methods. like smart folks we’ll only learn the most important ones. The important class that implements this interface is the TreeSet class. let’s look at the differences between List. The important class that implements this interface is the HashSet SortedSet represents unordered. Let’s first look at the ArrayList and Vector classes of List interface. ArrayList Following figure shows the structure of ArrayList data structure. Set and SortedSet interfaces. ArrayList and Vector implement List interface HashSet implements Set interface TreeSet implements SortedSet interface. we observe the following things: Collection is the top level interface.Chapter 7
Core Libraries and Best Practices
java. First.util Package
Collection
List
Set
SortedSet ArrayList Vector
HashSet
TreeSet
Fig 7. unique and unsorted elements. unique and sorted elements.1 Collection Hieracrchy
If you look at the above hierarchy. List represents ordered. Set represents unordered. duplicate and unsorted elements.

Chapter 7
Core Libraries and Best Practices
java. The above statement is perfectly legal and ‘list’ is an interface reference to the Arraylist object. 1. int size() : This methods returns the size the list. Display objects in the list The three important methods in this class are: void add(Object obj) : This method adds an object to the Arraylist. 2. is the following statement legal or illegal?
List list = new ArrayList(). Let’s do a quick example to get a complete picture of ArrayList class. ArrayList class implements List interface.util Package
0 object1
1 object2
2 object2
3 object3
4 object4
This class should be used for storing objects in an order like an array.4. Let me ask you a question here. Following are the basic operations we can do with this class. 3. This is the convention we normally use and is also one of the best practices. Based on this.4 (ArrayListDemo. public class ArrayListDemo { public static void main(String args[]) { // Create an ArrayList
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. See listing 7. If you notice the above class hierarchy.util.*. Create an ArrayList 2. There is no limit on the number of elements you can store. 1.
package utilities. Listing 7.
You are correct. Object get(int pos) : This method returns the object from the specified index. All the elements are indexed starting from 0. Add objects to the list 3. Following are the properties of this data structure. Allows storing duplicate objects.java) Using array list class. import java.

println(s1). This means. list.Chapter 7
Core Libraries and Best Practices
java. So. So. This is one of the widely used data structure in real world applications since we need to preserve the order of the data. The strings in the ArrayList are not automatically sorted. depending on what the context is. why do we need Vector when we already have ArrayList? This is an interview question. Since the get() method returns Object. It then used a for loop using the size() method to retrieve the strings back.add("World"). list. i++) { String s1 = (String) list. we need to narrow it down to String object since we stored strings.get(0). If this the case. i < list. list. The answer is. list. Vector If you look back at the class hierarchy diagram. System. you can use
196
.add("ABC"). Notice how we used the object casting within the loop. using Vector is safer in multi-threaded environments than using ArrayList.add("Hello"). // Add objects. // Get the objects for (int i = 0.add("World"). Compile and execute the above program to see the following output:
Hello World World ABC
If you noticed the above result.out. The string objects are displayed in the same order that we added. 2. right?. both ArrayList and Vector implement the same List interface. } } }
The above example code created an ArrayList and added four String objects using the add() method.util Package
List list = new ArrayList(). both of them will have same methods.size(). we can observe the following things: 1. keep this in mind. The list allowed duplicate objects (World) 3.

Look at the following figure to see how the objects are stored within HashSet. list.*.
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.
package utilities.java) Using vector class. } } }
If you noticed the above code.add("ABC"). list. // Get the objects for (int i = 0. list. Listing 7.util.add("World").5. list. The usage of Vector is exactly same as using ArrayList as shown in listing 7. public class VectorDemo { public static void main(String args[]) { // Create a Vector List list = new Vector().get(0). i < list.Chapter 7
Core Libraries and Best Practices
java.5 (VectorDemo.add("World"). One important thing with ArrayList and Vector is that the objects will never be sorted. the only difference is that we created Vector instead of ArrayList.size().add("Hello"). System.out. // Add objects.println(s1). The next couple of examples will demonstrate how we can automatically sort objects in a data structure. import java.util Package
either Vector or ArrayList classes. i++) { String s1 = (String) list. HashSet This class implements the Set interface and should be used when the order of the elements is not important.

boolean hasNext() : Returns true of there are more objects Object next() : Returns the object. It doesn’t store the elements in an order 2.5 and you’ll get the picture.java) Using hash set class. The three important methods in this class are: void add(Object obj) : This method adds an object to the HashSet. Use the iterator with a while loop to display the elements. However. we need to follow two simple steps listed below: 1. Listing 7.*. The Iterator class in turn has the following two important methods for retrieving the elements. to display the objects. we can add the elements using the add() method just like we did with the ArrayList and Vector classes. Take a look at the code in listing 7. public class HashsetDemo { public static void main(String args[]) {
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.util Package
Following are the properties of this data structure 1. 2. import java.Chapter 7
Core Libraries and Best Practices
java.
package utilities.6 (HashsetDemoDemo. It doesn’t allow duplicate elements 3. I know this is some what complex than array list and vector classes. Iterator iterator() : This method returns the Iterator object for the elements. It doesn’t sort the elements With this class. Get the iterator object.util.

a simple solution is that you add all the objects to HashSet and the size of the hashset gives you the count of unique elements. after adding the strings to the HastSet. we got hold of the iterator object using the following statement:
Iterator itr = set. then give me the next element”.add("World").iterator(). set.add("ABC"). // Get the iterator Iterator itr = set. // Add objects. set. while (itr. set.
We then used the two methods in the Iterator class with a while loop. set.
World ABC Hello
If you noticed the above result. For instance. 3.iterator(). we can observe the following things: 1.add("World"). The strings are not sorted. The working of the while loop is like. Are you with me? I am sure you are.add("Hello"). and displayed the elements.
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. you have to compare the current object with the previous object and all that crap.Chapter 7
Core Libraries and Best Practices
java. Strings are displayed in random order 2.util Package
// Create a Hashset Set set = new HashSet().next(). The hasNext() methods returns true until all the elements are returned.out. The only advantage with this data structure is that we can eliminate the duplicate elements.hasNext()) { String s1 = (String) itr. System. “If there are more elements. if you want to count the number of unique objects in a given set of objects. Without this. } } }
If you look at the above example. Compile and execute the above class to see the following result.println(s1). Didn’t allow duplicates when we added World twice.

Now let’s add the customer objects to the TreeSet as shown below:
TreeSet set = new TreeSet()."Edison".add(newCustomer("John".7a. public class LastNameComparator implements Comparator {
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.12345)).Chapter 7
Core Libraries and Best Practices
java.
package utilities. look at the following comparator class in listing 7. treeset expects us to pass an object of Comparator class that defines how the sorting should be done. and you’ll notice the strings sorted alphabetically as shown below:
ABC Hello Word
Let me ask you a question here. To better understand. the TreeSet will not sort the objects.
Based on the above rule. This makes sense because the treeset doesn’t know which particular field in the object to use for sorting. Let’s say we have a complex object (Java Bean) like Customer which encapsulates data like firstName. lastName and SSN. set. set. except that it sorts the elements in an ascending order."Smith".
Compile and execute the program with the above change. TreeSet expects the user to define how the sorting should be done.java) Comparator class. Rule: With String objects and Wrapper objects.*. With all other objects.add(newCustomer("Thomas"."Jackson". set.7a (LastNameComparator.456546)). TreeSet by default will sort the objects in ascending order.util. Look at the following rule. import java. right? In such situations.add(newCustomer("Laura". In the above program.util Package
TreeSet This class works just the same as the HashSet class. Listing 7.12345)). Does TreeSet sort any type of objects or just String objects? The answer is not that simple. change the following statement
Set set = new HashSet() to Set set = new TreeSet().

int ssn.util Package
public int compare(Object o1. This method takes two objects as parameters.compareTo(c2. Created a custom comparator class by implementing the Comparator interface.
public class LastNameComparator implements Comparator
2. We finally returned the index back.java) Customer class. this.Chapter 7
Core Libraries and Best Practices
java.firstName. 0 or -1 based on comparison implemented inside the method. we compared the last names of the two customer objects as shown below:
int index = c1.getLastName()). return index.lastName = lastName.firstName = firstName.compareTo(c2. this. 1. Implemented the compare() method. public class Customer { String firstName.
3.getLastName()).getLastName().7b & c shows the customer class and the test class. which should first be casted to the type of objects that we will store in the TreeSet and then return an int. Listing 7. we are now ready to use the TreeSet to sort the customer objects. Once we have the above comparator. String lastName. } public String getFirstName() { return this. public Customer(String firstName. } }
Here is what we did in the above code. int index = c1.ssn = ssn.7b (Customer. String lastName. Listing 7. Object o2) { Customer c1 = (Customer) o1. } public String getLastName() {
201
. Customer c2 = (Customer) o2. In our case.getLastName(). This method should return 1.
package utilities. int ssn) { this.

package utilities. set. import java. 78989)).add(new Customer("John".getFirstName() + " " + customer. 456546)). set. Once we passed this object.getLastName() + " " + customer. "Smith".next().7c (CustomerSortTest.util. Before it added the customer objects to the treeset object.java) Customer class.println(customer. System. } }
Listing 7.getSsn()).
Edison Jackson Smith Thomas Laura John 78989 456546 12345
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.lastName.*.iterator(). 12345)).add(new Customer("Laura".ssn.add(new Customer("Thomas". We can start using the TreeSet add methods as usual to add the different customer objects. Iterator itr = set. while (itr. public class CustomerSortTest { public static void main(String args[]) { TreeSet set = new TreeSet(new LastNameComparator()). it created a TreeSet as shown below:
TreeSet set = new TreeSet(new LastNameComparator()). the treeset knows how to sort the elements. } public int getSsn() { return this. "Jackson". set.util Package
return this.out.Chapter 7
Core Libraries and Best Practices
java.hasNext()) { Customer customer = (Customer) itr. "Edison". Compile and execute the test program and you’ll see the following result. } } }
Look at the CustomerSortTest class.
The rule is that we need to pass the object of the comparator class to the TreeSet constructor.

Use ArrayList to store objects when you want to preserve the order of the objects Use add() method to store the data Use get() method in a for loop to read the data Use HashSet to store objects when you don't want duplicate objects Use add() method to store the data Get the Iterator and use it with a while loop to read the objects. all we need to do is put a negative sign infront of index variable in the compare() method as shown below:
return -index. let’s see what they are.
Two Dimensional Data Structures
In two dimensional data structures. We need to iterate through the entire list and then get the object that matches the specified. This is where 2D data structures come in handy. that the customer objects are sorted by last name in ascending order. Following is the summary of 1-D data structures. They offer flexibility and faster way of retrieving the objects.util Package
You can see from the above result. You are good if you can just remember the above points. This completes all the one dimensional data structures.
Homework: Modify the comparator class to sort by SSN. Use TreeSet to store objects when you want to sort the objects Use add() method to store the data Get the Iterator and use it with a while loop to read the objects. Hint: The method should return +1 or 0 or -1 for greater than. If we want the lastnames sorted in descending order.Chapter 7
Core Libraries and Best Practices
java. objects are stored in the form of a table using keyobject pair as shown below:
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. equal to and less than comparisons.
Disadvantage with 1D data structures
The main disadvantage with 1D data structures is searching for a particular object. So.

we just have to learn how to use HashMap. In this category of classes. They are very simple to use. every data object (object 1. One more important thing is that we can only use these data structures only when every object we store has a unique key that identifies the object. Following is the hierarchy diagram for 2D data structures. let’s look at the 2D data structures one by one. object 2. Hashtable and Properties classes. Makes sense? In all the above three classes. So. Preperties class is a child class of Hashtable class. When a key is passed.
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. object3) is associated with a unique key.util Package
key Susan Tim Mac
Data Object object 1 object 2 object3
If you notice the above representation. There is no way you can store an object that doesn’t have a key.2 Map interface hierarchy
The top-level interface is the Map interface.Chapter 7
Core Libraries and Best Practices
java. Trust me. Hashtable and HashMap are the two classes that implement this interface. the data structure will return the data object corresponding to this key. right? This is why searching is fast with 2D data structures. one for storing the object and the other for retrieving the object. With these things in mind. the data object can be retrieved in just ONE step. given a key. there are just 2 important methods we need to know.
Map
Hashtable
HashMap
Properties
Fig 7.

util. we need to first identify the unique key associated with each customer. // Create a hashmap HashMap map = new HashMap(). c3). 456546). we can use SSN as the key. c1). Normally.util Package
HashMap A Hashmap object stores the data in the form of key-value pairs.getLastName() + " " + customer. 78989). In our case. Both the key and the value can be object. Take a look at the code in listing 7.getFirstName() + " " + customer.
205
.8 (HashMapDemo. We first created three customer objects namely c1.put("345678". Customer c3 = new Customer("Thomas". map. Let’s use the same customer class we wrote in the previous example and use a HashMap to store different customer objects. } }
Look at the above class.put("12345". "Edison".8.out. the put method will be as shown below:
map. lastname and ssn.java) Class using Hashmap
package utilities. "Smith". c3 by supplying all the data. c2. Customer c2 = new Customer("Laura".getSsn()). "Jackson".Chapter 7
Core Libraries and Best Practices
java. c1). This method takes the key as the first argument. Since we decided to use ssn as the key. map.put("98765". Let’s say we wish to store data about several customers who has firstname. 12345). c2). To store this data. We then created a HashMap object and used the put() method to store the data.put("12345 ". System. // Store the data using ssn as the key map.*. a String object is used as the key to store any other objects. and the data object as the second argument. Listing 7. public class HashMapDemo { public static void main(String args[]) { // Create 3 Customers Customer c1 = new Customer("John".println(customer. // Get Smiths info Customer customer = (Customer) map. since it is unique.get("345678"). import java.

which is the ssn. we need to do the object casting here to get the required customer object as shown below:
Customer customer = (Customer) map. while HashMap cannot. we simply displayed its properties. we use getProperty() and setProperty() methods. but the only difference is that we use it in situations where both the key and value is a String object.get("345678 "). Note that. Take a look at the code in listing 7. In most of the real world applications we use HashMap instead of Hashtable. But it’s a good practice to always use String object as the key to all the data objects. With this class.
Once we got the customer object. This is one class that is widely used in real world applications. This method takes the key. the data objects that we can store can be any object. The answer is again the same as with ArrayList and Vector.9.util Package
Once we stored all the three customers data. Note: If you look at both HashMap and Hashtable classes. It can be Customer object. In the above program. Properties This data structure works much the same as HashMap. or Box object or anything else.
Laura Jackson 456546
Hashtable Hashtable works much the same as HashMap. Compile and execute the class to see the following result. then why do we need Hashtable? This is yet another interview question.
If you recompile and execute the above class. just change the following statement
HashMap map = new HashMap() to Hashtable map = new Hashtable(). If this is the case.Chapter 7
Core Libraries and Best Practices
java. we used the get() method to retrieve the data. the output will still be the same.
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. and returns the customer object associated with that key. instead of using get() and put() methods like in HashMap. Hashtable can be safely used in multi-threaded environment.

util Package
Listing 7.10. p.
package utilities.
package utilities.9 (PropertiesDemo. public class DateDemo { public static void main(String args[]) { Date d = new Date(). we used setProperty() method to which we passed string as both key and data. Let’s now look at two more important utility classes that are widely used while representing and processing dates. returns the String object and we don’t have to do the casting. Look at the code in listing 7. Compile and execute the above class to see the following result:
localhost:8080
This completes all the important data structures that we need to know.*.println(str).Chapter 7
Core Libraries and Best Practices
java.
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.test. Date class This class is used to get the current system date. We then used getProperty() method by passing the key and reading the string value back. import java. p. System. The usage is pretty simple. Listing 7.out. public class PropertiesDemo { public static void main(String args[]) { Properties p = new Properties(). } }
As you can see in the above example.*.java) Class using Date class. String str = p. "com. import java.getProperty("url").java) Class using properties.10 (DateDemo. Most of the real world applications don’t go beyond the above mentioned data structures unless otherwise the requirements are highly complex. The getProperty() method instead of returning Object like the get() method.Driver").util.setProperty("url". "localhost:8080").util.setProperty("driver".

This method will then return the date as String object in the desired format.
Following is what we did. } }
The above program simply created a Date object and displayed it. the Date object displays both date and time. the question is. convert it to Date object. SimpleDateFormat This class belongs java. Case 2: Given a date as String.format( new Date() ). how do we format dates? The answer is using SimpleDateFormat class which is the next class we are going to discuss. let’s see the following two cases.util package.
SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy ") String formattedDate = sdf.text package. If you compile and execute the program.
SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy "). To better understand how this class works. Case 1: Convert the given Date object into String in the form of MM/dd/yyyy format.util Package
System. to format and parse the dates. We use this class. dd as day and yyyy as year. Following is how we achieve this. Though this is not harmful.println(d). Createed a SimpleDateFormat object with the format of the date string
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. sometimes in real world applications. we require the date to be displayed in different formats. So.parse("03/12/2005 "). We then have to invoke the format() method by passing the Date object that is to be fomatted. we first have to create SimpleDateFormat object by passing the format that we need.Chapter 7
Core Libraries and Best Practices
java.out. JVM will interpret MM as month. Following is how we parse the dates. you’ll see the date displayed as shown below:
Tue Jan 13 12:34:789 2006 EST
If you notice the above result. For instance like MMddyyyy or yyyy/MM/dd etc.
As you can see from the above statements. not the java. This is the reverse of the above and we call it as parsing the date. Date d = sdf.

11.
209
.
package utilities.format(new Date()). to Date d = sdf.println(formattedDate). import java.*. try { Date d = sdf. This parse() method returns the constructed Date object from the string.util. System.*.
You’ll see the message “Unable to parse Date”.11 (SimpleDateFormatDemo. // Case 2 sdf = new SimpleDateFormat("MM/dd/yyyy"). This is a checked exception and we need to enclose the method call in a try-catch block.println(d). } } }
Compile and execute the above program to see the following result:
03/12/2006 Tue Mar 03 12:34:789 2005 EST
Try changing the following statement
Date d = sdf.Chapter 7
Core Libraries and Best Practices
java.parse("03/12/2005"). as it throws exception due to improper format.text. Just in case if it fails to parse the date. } catch (ParseException pe) { System. See the code in listing 7.util Package
Passed the date to the parse() method. import java.parse("2005/03/12"). String formattedDate = sdf. it will throw a ParseException that should be caught.parse("03/12/2005").java) Class using date formatter class. System.out.println(" Unable to parse Date"). Listing 7.out. public class SimpleDateFormatDemo { public static void main(String args[]) { // Case 1 SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy").out.

consider the following text data.nextToken(). Firstly.abcd. import java.Y. This is when we use the String Tokenizer class.abcd. This class has two methods namely hasMoreTokens() and nextToken(). It tokenizes the original text and gives us individual tokens.
StringTokenizer
This class is a cool class that breaks the text using a delimiter.").abcd.").12 (StringTokenizerDemo. ".98765.java) Using string tokenizer class.
We then have to use the hasMoreTokens() and nextToken() methods to get the individual strings (tokens).Chapter 7
Core Libraries and Best Practices
java.println(token).Y.ABCD ".ABCD".util. while (st. public class StringTokenizerDemo { public static void main(String args[]) { String str = "12345. To better understand. System.
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package utilities. Listing 7. For instance. ". The description for these two methods is given below: boolean hasMoreTokens() : This method returns true if there are more tokens left.Y.util Package
Let’s see one last class called StringTokenizer.ABCD ".98765.98765.
Let’s say that someone gives us a comma delimited text in the above form.12. StringTokenizer st = new StringTokenizer (str .StringTokenizer. Let’s see how to use this class.hasMoreTokens()) { String token = st. String nextToken() : This method returns the actual token. and we are required to break it into pieces at the delimiter.out.
String str = "12345. we need to create an object of the class by passing the original text and the delimiter where we want to break it as shown below:
String str = "12345. look at the code in listing 7. StringTokenizer st = new StringTokenizer(str. which is also used very frequently in real world applications.

Compile and execute the program to see the following result. src. conventions that we usually follow in al the real world Java applications. parseNameLine etc.
12345 abcd Y 98765 ABCD
This are all the utility classes that we need to know to write any complex Java programs. Class names must always start with a uppercase letter.
Naming Conventions
Using meaningful names in programs improves the readability of the program. BeanUtilityFactory etc.Chapter 7
Core Libraries and Best Practices
Naming Conventions
} } }
The while loop works like. that’s OK as long as you know how to use them. setName. 4. Method names must always start with a lowercase letter. be cool. 3.utilities etc. Package names must be in lowercase. So. Most of the real world applications use these classes to simplify the development. then give me the next token”. Following are the naming conventions we should follow while writing programs: 1. Any method that returns a boolean should start with “has” or “is” as shown
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. Even if you couldn’t remember the method names. We don’t want to write a program in such a way that people who use the program have a tough time understanding what the program is all about. com. As an example. BusinessDelegate. and every word in the class name must also start with an uppercase letter as shown below: InitialContext. ”If you have more tokens. The next few pages give you an insight of some of the best coding practices. and every other word in the method name must start with an uppercase letter as shown below: getName. 2. Just learn them once.

but not the public users. which is what we’ll see in this section. Moreover. Note: Writing a Java program is not the key here. 6. static final String PROVIDER_URL = "localhost:8080". let’s also be on the same page with them. we need high-level documentation.
Java Documentation
Documentation of the program is the most important thing that is very strictly enforced in all the real world applications. there will be dedicated personal who strictly enforces the usage of the above standards. how will the users of the program know how to use it? Documenting a Java program is different from using comments in a program. If there is no documentation. these conventions are followed by every one in the Java community. In every single Java project. and should always be in uppercase with words separated by an underscore. A program without any documentation is simply of no use at all. Trust me. Constant names must always be declared as static and final. boolean hasStatments. So. Wring a good Java program following the best practices and coding conventions is the key.Chapter 7
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Java Documentation
below: hasMoreElements() . static final String VAL_CHECKING = "Checkings". Using comments is a low-level documentation that aids the programmers. When somebody looks at our program. We use comments inside the program to make the program self-explanatory.
static final int MAX_QTY = 100. ‘status’ etc. hasPermissions() etc 5.
Avoid using names like ‘flag’. as shown below:
boolean isMarried. So. reading the comments must tell that what the program
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. We write programs so that someone uses them. For public users. isNameValid(). All the boolean variable names should start with the words “is” or “has”.
Always follow the above rules while writing the Java programs. it’s always good that we learn to use them right from the beginning. This will really help you in making your life easy.

In order to generate the above documentation for a class. it’s time to learn how to generate high-level documentation for a Java class. 3. what interfaces it implements. */ Now. what exceptions does the method throws. We already wrote several programs using comments as shown below: // This is a single line comment /* Following statements uses the SSN to check whether or not the candidate is eligible for a Credit Card. we need to first write the descriptions in the class using javadoc tags. 2. 1. what child classes it has and so on. Lists all the method names and a description on how to use the methods. This tool generates the following documentation for a given class. The comments must be written in such a way. Class details like who the parent class is. that even non-java programmers must be able to understand the program.Chapter 7
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Java Documentation
is all about. what values it return back and all that good stuff. Good. We use a tool called javadoc to generate this documentation. Following table lists the six most important javadoc tags. These are very simple to use. and you can master them in few minutes. Trust me. Table 7. so that you can also publish it online. All the documentation will be generated in HTML format.
Documentation using javadoc tool
javadoc is a documentation generating utility tool that comes along with the java installation. Lists all the global variables and constants. that you’ll se surprised after seeing it. The documentation that the javadoc tool generates is so cool. this is very simple.2 Javadoc tags Tag Name @author @since @version Description Used to specify the name of the person who wrote the class Used to specify the creation date of the class Used to specify the version of the class
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. what parameters it takes.

1234 Broad St. import java.io.java is the program we wanted to generate the documentation and
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.0 */ public class CustomerGateway { // Global Variables private static final String MAX_ACCOUNTS = 3.*.util. import java.*.
package utilities. Used to specify the name of the person who wrote the class
The best way to learn the usage of the above tags is by writing a program. javadoc is the tool name utilities\CustomerGateway. Look at the following example that uses the javadoc tags.Chapter 7
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Java Documentation
@param @return @throws @author
Used to specify the method parameters Used to specify what the method returns Used to list all the exceptions a method throws. * * @author Steve * @since October. 2006 * @version 1. NY". } }
Copy the above program into the “utilities” directory and execute the javadoc command as shown below:
C:\JavaTraining\Chapter6>javadoc utilities\CustomerGateway.java –d docs
In the above command. * * @param ssn – The SSN of the Customer * @return String – The profile of the Customer * @throws IOException – Thrown when Customer is not found */ public String getCustomerProfile (String ssn) throws IOException { return "John. /** * Retrieves the Customer profile based on SSN. /** * Gateway class that has several methods to retrieve customers * information.

The page displays all the details of the class. it is an automatic child of Object class as shown. If you open this file. This information is specified in our class right before the class name as shown below:
/** * Gateway class that has several methods to retrieve customers * information. 2006 * @version 1. and its inheritance hierarchy. you’ll see the following:
The index.0 */ public class CustomerGateway {
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. the author name.html list the class as shown above. Click on the class name. Since the class doesn’t extend from any class.html.Chapter 7
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Java Documentation
-d docs specifies the directory in which we want to store the documentation The above command generates several HTML files and stores them in the docs directory. The starting point of the documentation is index. * * @author Steve * @since October. creation date and version. It has four sections as listed below: Section 1 lists the name of the class. and it displays the content shown in Fig 7. Section 2 displays the class description.3 in the next page.

the method parameters and the return type. they would be displayed too. if you defined your own constructors. This is what is shown here. If you click on the link.3 Class Details Page
Any Javadoc comment will have a short description followed by the tags. as we learned before. However. JVM will supply with a default constructor. It will display all the methods in the class as hyperlinks along with a description. Section 4 gives the method summary of the class.Chapter 7
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Java Documentation
Fig 7. Since we didn’t define any constructor. All the javadoc comments must be enclosed within /** and */ and every line must begin with a * as shown above. it will show you the complete details of the method as shown below:
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. Section 3 displays the constructors of the class.

* * @param ssn – The SSN of the Customer * @return String – The profile of the Customer * @throws IOException – Thrown when Customer is not found */ public String getCustomerProfile (String ssn) throws IOException
Isn’t the documentation beautiful? Are you surprised? I am sure you are. in real world applications there will be tons of classes in a single package. it displayed the entire method signature with the following items: 1. The description of the method. there will be several methods. In the above example. 4. The good thing is that we just have to specify the information in the program.
All the above 4 items came from the following chunk in our code:
/** * Retrieves the Customer profile based on SSN. and you can use the following command to generate documentation for all in one step as shown below:
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. and the javadoc tool will take care of the rest.Chapter 7
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Java Documentation
If you notice the above figure. we just generated the documentation for one class. However. and you need to follow the same procedure for all the methods. 3. The parameters to it The data it returns back All the possible exceptions it can throw. In real world applications. 2.

java –d docs
The above command generates the documentation for all the java files in the utilities package. StringBuffer class must be used in situation where complex text processing is required. so let’s summarize. StringTokenizer class is used to break a large string into smaller strings using a delimiter. A program can import any number of classes and packages. and also has several methods for text processing. All we did is. Long etc represent the corresponding primitive variables as objects. I am sure you enjoyed this chapter as much as you did with the previous chapters.
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. duplicate and unsorted objects.Chapter 7
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Summary
C:\JavaTraining\Chapter7>javadoc utilities\*. String. The two most important packages are java. and stores it in the docs directory. StringBuffer and StringTokenizer. The classes in java. learned some library classes that are frequently used in real world applications and more importantly the best practices and coding conventions. This completes all the important core classes that I’d like you to know to be more comfortable with Java programming. String class is the widely used class as it represents text data. ArrayList and Vector are the two important classes that implement List interface.lang package are wrapper classes. List and Set interfaces represent 1D data structures. The only disadvantage with String class is that JVM creates a completely new String object when the text is modified.util. we first need to import the package using the import statements. Float. To use a library class in a package. Double. Data Structures are broadly classified as one-dimensional and teodimensional. These classes are used to store data objects. List interface represents ordered. This is all you need to know about the javadoc tool and the tags to use in the program to specify the descriptions.
Summary
Java comes with rich set of built in library classes organized in packages. The important classes in java.lang and java. Wrapper classes like Integer.util package forms the Collection Framework.

Which of the following is a wrapper class? a) integer b) Integer 2. Which of the following are the methods in StringTokenizer class? a) hasNext() and next() b) hasMoreTokens() and nextToken()
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. Use SimpleDateFormat to format and parse dates. Map interface represent 2D data structures. Data is stored in the form of table using key-object pairs.Chapter 7
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Summary
Set interface represents unordered. but it sorts the objects in ascending order. Which of the following is a method in String class? a) substring b) substring 3. TreeSet class is same as HashSet class. Properties object is used when key and object are Strings. Using javadoc tags is a best practice and is always enforced in all the real world applications. Use HashMap instead of Hashtable in thread-safe environments. Use Date class to display dates.
Time to play 50-50
1. javadoc tool is used for generating documentation for classes. HashSet is one important class that implements the Set interface. To use the tool. HashMap and Hashtable classes implements Map interface. A good Java program must always follow the best practices and coding conventions. we first need to write the javadoc tags in the program. Which of the following should be used for breaking the Strings? a) StringBuffer b) StringTokenizer 4. unique and unsorted objects.

Which of the following class implements List interface? a) HashMap b) ArrayList 8. Which of the following is used to store UNIQUE elements? a) ArrayList b) HashSet 9. Which of the following interface represents ORDERED elements? a) Set b) List 7. Which of the following are methods in Properties class? a) getAttribute() and setAttribute() b) getProperty() and setProperty() 13. Which of the following sorts the elements in ascending order? a) HashSet b) TreeSet 10. Which of the following class is used to display dates?
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. Which of the following is the top-level interface in the Collections framework a) Collection b) Object 6. Which of the following are methods in Hashtable class? a) get() and put() b) getAttribute() and setAttribute() 12.Chapter 7
Core Libraries and Best Practices
Summary
5. Which of the following class stores the data as key-value pairs? a) HashSet b) HashMap 11.

StringBuffer is a mutable object and JVM will use the same object while the contents are modified. Which of the following class should be used to format and parse dates? a) Date b) SimpleDateFormat 15. Answer: ArrayList. Properties. Question: What is the difference between ArrayList and Vector class? Answer: Vector is thread safe than ArrayList since the methods in Vector are synchronized. Vector. This is also the reason why StringBuffer offers better performance than String class when complex text processing is involved. Which of the following checked exception is thrown by the parse() method in the SimpleDateFormat class? a) ParseException b) ParsingException
Interview Questions
Question: What is the difference between String and StringBuffer classes? Answer: String is an immutable object which means that JVM will create a new string object when ever the contents are modified. Which of the following are the methods in SimpleDateFormat class? a) format() and parse() b) formatDate() and parseDate() 16. HashMap.Chapter 7
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Interview Questions
a) Date b) SimpleDate 14. HashSet. TreeSet. Question: List some of the collection data structures in java. Question: What is the difference between HashMap and Hashtable?
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.util package. Hashtable.

Using Properties. while casting data is required with Hashtable. This is because all the methods in Hashtable are synchronized by default. So. I sugget you to take few minutes to read the next two very small chapters.
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.Chapter 7
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Interview Questions
Answer: Hashtable is thread safe than HashMap. You know what. Question: Which class is used to format dates? Answer: SimpleDateFormat Question: What is the difference between Hashtable and Properties? Answer: Properties is a child class of Hashtable which is used when both the key and value are Strings. let’s rock and roll. The second difference is that. while Hashtable doesn’t. However. this chapter is an unofficial end to the Java fundamentals section. we don’t have to cast the data. This is the end of this chapter. HashMap allows null keys and values.

you’ll understand the basics of how multi threading is done.Chapter 8
Threads
Chapter Goals
Chapter 8
Threads
This chapter will introduce you to concurrent processing in Java. By the end of this chapter.
C:/JavaTraining/Chapter8/threads threads is a package in which we will store the programs. Understand the life cycle of threads Understand synchronization
Environment Setup
All the programs in this chapter should be stored in the following directory.
move to the following root directory and get ready. To compile the programs.
C:/JavaTraining/Chapter8
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.
Chapter Goals
Understand the importance of concurrent processing Understand the API for multi-threading. Java language has built-in support for writing programs that run simultaneously.

Web application etc. computers have gone through radical improvements using sophisticated semiconductor technologies and today we are in the world of Supercomputers that compute trillions of instructions per second. So. We see multi-processing when ever we use a computer. We open a word document. For instance word processing application. Creating the processor is the job of semi conductor industry. Once we have super fast processors it is very important to use them to their full potential. A processor is a hardware device in a computer that processes the instructions.
Multi-Processing
Multi-Processing is the ability of CPU to concurrently execute multiple processes at the same time. our primary goal is to improve the “speed” with which computing is done. Let’s start with the term “processor”. For instance a grammar check thread in a word processing document. right? Some operating systems also allow us to monitor the processes that are running on the computer. a web page and many more applications and use them simultaneously. Based on the type of the
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. we need to know something called “Thread”. This technology helped us to build super fast processors for computing. There is also something called “multi-threading” which is often confused with “multi-processing”. Using the processor effectively and efficiently is the job of the operating system of the computer.. what is a process? A process is nothing but any currently running application. Email application.Chapter 8
Threads
Introduction
Introduction
Ever since the invention of first computer named Abacus to the latest and super fast computers. So. Over the years operating systems too have undergone significant changes incorporating better algorithms to better utilize the CPU and one thing that contributed significantly in this space is the “multi-tasking” or “multi-processing”. based on the need for high speed computing. A thread is nothing but a sub process that runs within a process. All I mean to say is that. Let’s see what these are. both these are completely different but they are related with other. for faster processing we need super fast processors. So.
Multi-Threading
Before we understand what multi-threading is. This is where the silicon technology comes into picture. To tell you. how is this achieved? Good question. using a super fast processor effectively and efficiently is more important than just having the processor. Over several years. right? There is no point in having a supercomputer and making it sit idle for most of the time waiting for something to happen.

This is what we call as multithreading. every process will have its own threads and will never interfere with another process threads.
How Multi-Threading is done?
This is a big question. Following picture should help you understand the difference between multi-processing and multi-threading. multi-threading is the ability of the CPU to execute multiple threads within a “single process” simultaneously. To understand this clearly. In this chapter. So. it can have any number of sub-processes (threads) and it may require that these threads be running concurrently within the process. look at the following figure.
Process 1 threads Process 1
Process 2 threads Process 2
Process 3 threads Process 3
Operating System
As you can see from the above figure.Chapter 8
Threads
How Multi-Threading is done?
process. we will see how we can use Java to work with threads and how to build multi-threaded applications.
Threads Creates Java Application/Process
Scheduler Thread Queue
CPU
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.

Once it does all these actions. Following figure should give you some idea on how the scheduler manages the threads. the scheduler will send a thread to the CPU for a small duration of time (called as time slice). Most schedulers follow “time-slicing” algorithm for managing threads. Once the time slice is completed. With this algorithm. there are several components that participate in multi-threaded programming.
Thread Queue
This is a component that will contain all the threads that are created by the above Java application and also the threads from other processes.
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.
JavaApplication/Process
This is the application that we as Java developers write using the multi-threading features of Java language. There will not be a separate queue for every process. it is important to understand that this behavior is never guaranteed. its job is done.
Scheduler
This is the component that manages the threads. it will resume executing from where it stopped before. The scheduler can send a low priority thread to the CPU ahead of a high priority thread. The scheduler component comes with the operating system.Chapter 8
Threads
How Multi-Threading is done?
As you can see from the above figure. However. The scheduler acts like a manager and decides which thread should be sent to the CPU based on things like priority and some thing else. There is nothing it can do or control how the threads should run or when they should run and all that stuff. set the priorities to threads and start the threads. If there is a thread in the queue with high priority sitting behind than the lesser priority one. it may pick the highest priority thread and send it to the CPU. This switching between the threads by the scheduler will be so fast that from the application point of view it looks like all the threads are running simultaneously. is pulls the thread out of the CPU even if it is partially executed and puts it back in the thread queue. All the threads will be lined up in this Queue waiting to get into the CPU. When a previously pulled out thread gets a chance again to go to the CPU. Following explains the roles and responsibilities of each of the above components. It then sends the next thread in the queue to the CPU and again pulls it back after the time slice. Its main responsibility is to create threads.

the scheduler makes sure that all the threads are executed completely by the CPU. let’s understand the life cycle of a typical thread. If the thread is completely executed.
Yield
Start
Runnable
Running
Terminate
Sleep
As you can see from the above state diagram. it will or can go to the runnable state again in the following 3 ways. Otherwise. 1.Chapter 8
Threads
Thread Lifecycle
Thread 1
Thread 2
Thread 3
Thread 2
Thread 1
Thread 3
Thread 4
Thread 4
1 time slice = 1ns
As you can see from the above figure. By the end of the cycle. This is the state of the thread in the queue and is ready to be executed whenever the scheduler decides. it goes right away to the runnable state. From the runnable state. every thread will get a 1 slice of CPU time at a time. then from the running state it directly goes to the terminated state which is a dead state. The time slice will normally be in nano and pico seconds based on the CPU frequency which cannot be recognized by the naked eye and this why we feel the simulation of multi-threading or concurrency behavior. it goes to the running state. A running thread can voluntarily yield or relinquish the CPU and go back to the runnable state.
Thread Lifecycle
Following are the states a thread can go from the creation to its death. once the thread is started. This is normally done by the thread itself when ever it thinks that
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. This is the state when the thread is inside the CPU. Now that we know how multi-threading is implemented internally.

you are always guaranteed that the next thread comes to the CPU only after t1 sec where t1 is the amount of time the thread slept. Now that we know the typical lifecycle of a thread.Runnable interface 2. If however there are no other threads in the queue. java. 2. A running thread can be normally brought to the runnable state by the scheduler. These are 1. These are good threads because they care about other threads. A running thread can sleep for a definite amount of time and then go to the runnable state.
Multi-Threading in Java
Java language supports multi-threading using the standard built-in classes.1 Thread API Method Name String getName() int getPriority() boolean isAlive() void run() void start() void sleep() void setPriority() Description Returns the name of the thread Returns the priority of the thread Determines if the thread is running or not Entry point of a thread Method to start the thread Makes the current thread sleep of certain duration of time Sets the priority
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.
Thread class
Table 8. In this case. let’s see how we can do multithreading in Java. Usually threads will sleep because their operation is dependent on the execution of other threads.lang.Thread class Following table lists the important methods in each of the above. The good thing is that there is just one class and one interface we need to know to start writing multi-threaded applications.Chapter 8
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Multi-Threading in Java
its job is not a high priority one.lang. java. 3. the scheduler will send it back to the CPU again.

currentThread().out. the class that has the main method is itself the parent process and we call it as the main thread. before we start creating the threads. the question is. So.
package threads. who is this parent process? You know what. Therefore. It is this parent process that creates the threads. So.1 (TestThread. What if there is no parent process? The answer is simple. System.getName()).println(t. Let me ask you a question. Take a look at the code in listing 8.println(t. The currentThread() method returns the Thread object that is actually running the
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. } }
Compile and execute the above program to see the following result. the name of the thread is “main” which is the parent thread and its priority is 5.Chapter 8
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Multi-Threading in Java
static Thread currentThread()
Returns the current thread
Runnable Interface
The only method in this interface is the “run” method as shown below: public void run(). threads are nothing but sub processes within a parent process. public class TestThread { public static void main(String args[]) { Thread t = Thread.out. let’s prove this fact by writing a simple program. we need to have a parent process. we will start spawning threads.
main 5
As you can see from the output.java) Main thread class. It is within this main method. System.getPriority()). Listing 8.
Main Thread
As I said before.1. There will not be any threads. This is the default priority of the main thread.

In our case it returned as the main thread.java) Using Runnable Interface
package threads.println("Executed the thread").2a (MyThread.
Creating a Thread
In Java. You might wonder why we need two different ways of creating a thread. Start the thread. 1. See the following code in listing 8. This is the rule. Implementing the Runnable interface 2. We’ll get to the answer later. take a look at the test class in listing 8. Let’s look at mode details on threads from here onwards.
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. there are two ways of creating a thread as shown below: 1. 2. Extending the Thread class. Let’s first understand how to create the threads using both ways.Chapter 8
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code. we first wrote a thread class that implements the Runnbale interface with the run() method. public class MyThread implements Runnable { /* * Any statements that should be executed concurrently * must go in this method.out. } }
As you can see from the above code.2b that creates and starts this thread. Create a Thread object by passing the Runnable object 3. Listing 8.2 that demonstrates the above three simple steps. Runnable Interface Creating a thread using a runnable interface involves 3 simple steps. Ok. All the statements that should be executed concurrently must be placed in this method. Now. Write a class that implement the Runnable interface and implement the run() method. This proves that main method itself is the parent thread. * */ public void run() { System.

Chapter 8
Threads
Multi-Threading in Java
Listing 8. The rule is. place all the statements that are to be executed inside the thread in the run() method. // Step 3 th. and passed it to the Thread object constructor. One thing you need to always keep in mind is that with multi-threading the output is never guaranteed to be same on all computers or even by running the same program again an again. followed by MyThread. } }
In the above test class. // Step 2 Thread th = new Thread(t1).println("Main Thread about to terminate"). the output statements may be reversed.println("Main Thread terminated"). System.2b (RunnableTest.java C:/JavaTraining>chapter8>java threads.out. When the start() method is invoked. When the CPU executes the thread (which is MyThread) . the main method (parent thread) created the object of MyThread class. it executes the statements in its run() method.
package threads.out. you can see that it executed the main thread first. Compile and execute the programs as shown below:
C:/JavaTraining>chapter8>javac threads\*. Finally it started the thread using the start() method. public class RunnableTest { public static void main(String args[]) { // Step 1 MyThread t1 = new MyThread().
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.RunnableTest
The result of the above test class may be the following:
Main Thread about to terminate Main Thread terminated Executed the thread
From the output.start().java) Test class creating a runnable thread. This is because every OS will have its own scheduler program that functions differently from others. System. the thread will go to runnable state (queue) and is now at the mercy of scheduler. On some other computer.

You as a developer can only suggest but not command the scheduler.
where
MIN_PRIORITY = 0 NORMAL_PRIORITY = 5 MAX_PRIORITY = 10
In the previous program. ok. But again. t.setPriority(Thread.setPriority(Thread.start().3. the scheduler decides whether or not to obey this order.
Compile and execute the program again with the above statement and see if the output is reversed or not. t.MAX_PRIORITY).setPriority(Thread. See the code in listing 8.MIN_PRIORITY).
package threads. public class AnotherThread extends Thread { public void run() { System. th. we need to write a thread class that extends the Thread class and overwrite the run() method.setPriority(Thread.out.3a (AnotherThread. Listing 8. }
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. There are three priorities we can set on a given thread such as low.java) Thread class.
MyThread t1 = new MyThread().println("Executed the thread"). th. Thread th = new Thread(t1). The result can be something like shown below:
Main Thread about to terminate Executed the thread Main Thread terminated
Creating a Thread using Thread class
In this case. Thread priority gives the programmer a way to only suggest the scheduler to run the thread ahead of other low priority threads. we can also set the priority of a given thread. medium and high as shown below:
t.Chapter 8
Threads
Multi-Threading in Java
Thread Priorities In Java.MAX_PRIORITY). let’s set a high priority as shown below to see if our thread get’s executed before the main thread. This is just a different style.NORMAL_PRIORITY).

Start the thread. to create a thread all we need to do is.out. System. right? Let’s now write the main class that creates the above thread. Most applications uses this style of creating threads.java) Test class for creating a thread. When do we use Thread class and when do we use Runnable interface? Answer: Use implements Runnbale style when the thread class should also be a child class of some other non-thread parent class as shown below:
public class MyThread extends SomeNonThreadParent implements Runnbale
If however the class doesn’t have to inherit from any other non-thread parent class. then use the extends Thread style.Chapter 8
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}
As you can see.
package threads. t1.3b (ThreadTest. Create the object of thread class and 2.println("Main Thread completed"). Now. } }
As you can see from the above test class. This is the simplest way of creating a thread. 1. let’s find out the answer to the question we left couple pages ago. Easy.
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. Listing 8. we created a thread class by inheriting from Thread class as shown below:
public class AnotherThread extends Thread
We then overwrote the run() method.start(). public class ThreadTest { public static void main(String args[]) { AnotherThread t1 = new AnotherThread().

D. if the thread gives you some alert at regular intervals of time whether or not to continue further.B.4 shows the code for low priority thread and high priority threads.println(getName() + " yielded"). System.Chapter 8
Threads
Multi-Threading in Java
Thread Yielding
A thread can voluntarily give up the CPU to give other threads a chance to get into the CPU. in such cases it would be beneficial to make the thread yield at regular intervals of time so that we can take decision whether or not we want to proceed further or terminate the thread. } public void run() { System.out. public class LessImportantThread extends Thread { public LessImportantThread(String name) { super(name).4a (LessImportantThread. yield().sleep(1000). In such cases graciously giving up the CPU for other threads that are deemed to be of utmost critical is a good thing.C. } catch (Exception e) { } System.out. So why should the thread so graciously yield the CPU? Usually when we have a threads that need to run for long duration of time say for several hours. and priority 10 for B. then based on the importance of the job you can decide to either kill it or run it.D making them more critical than A.out.C. As an example. Let’s write a small example where we have 4 threads namely A. } }
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. you have to patiently wait for 4 hours before you leave home which I know is not good at all. What we will do is.java) Low Priority Thread
package threads. try { Thread.println(getName() + " Running"). On the other hand. Ok. Listing 8. If it doesn’t yield. Listing 8. let’s say your notebook started running a background process to backup your hard disk and takes like 4 hours to complete the job. set a priority of 5 for thread A.println(getName() + " Running Again"). Another typical application is the threads that do less important work.

}
As you can see from the above code. } public void run() { System. public class ThreadStarter { public static void main(String args[]) { LessImportantThread t = new LessImportantThread("A"). The Thread class has a method called getName() which when used.
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.
package threads.setPriority(5).Chapter 8
Threads
Multi-Threading in Java
Let’s look at the run() method in the above LessImportantThread class. When it yielded.println( getName() + " Running Again").out.4c (ThreadStarter) Main thread class spawning multiple threads. public class CriticalThread extends Thread { public CriticalThread(String name) { super(name).out. will return back the thread name.
public void run(){ System. the thread yielded by calling the yield() method.out.4b (CriticalThread) High priority Thread
package threads.println( getName() + " yielded"). let’s now look at the ThreadStarter parent thread class. t. This method inherited from the Thread class. System. Listing 8.start(). t.println(getName() + " Running"). System. } }
The run() method in the CriticalThread class is pretty simple. With the above two thread classes. Listing 8.out.println( getName() + " Running"). yield(). Its constructor takes a name argument an passes it to parent class constructor. the scheduler may give the other threads a chance to execute because they are set high priority.

ThreadStarter
The result of the above program may be as shown below:
B Running A Running A yielded C Running D Running Main Thread completed A Running Again
Looking at the above result. t3.setPriority(10).
To display the thread name for debugging purposes. So the solution to keep the toy
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.D with priorities 10.start(). t2. t2.
The above class simply created one less useful thread with the name A and priority 5.C. This scenario can be viewed as four kids fighting for the same toy and you all know what will happen to the toy ultimately. t1.start(). we can call the getName() method which prints the thread name. One of the widely used application of threads is the resource sharing where multiple threads compete for the same resource at the same time.out.Chapter 8
Threads
Multi-Threading in Java
CriticalThread t1 = new CriticalThread("B"). Finally our less useful thread A came back and finished its work. t3. when A yielded. our scheduler sent C and D threads to the CPU followed by main thread. Usually the thread name should be passed to the Thread class which is why we used the following statement in the thread constructors:
super(name). t1.start(). Compile and execute the above classes as shown below:
C:/JavaTraining>chapter8>javac threads\*.setPriority(10).setPriority(10). and three critical threads with names B. This is when the resource will get messed up. CriticalThread t2 = new CriticalThread("C").println("Main Thread completed"). } } System. CriticalThread t3 = new CriticalThread("D").java C:/JavaTraining>chapter8>java threads. This is exactly what we did in the run() methods.

println("Printing " + data + " completed").1 Multiple threads accessing the same resource
Listing 8. We will look at two cases.5a (Printer) Resource Class
package threads. public class Printer { public void print(String data) { System. } }
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. In our example we will use a printer object as the resource that multiple threads use to print the documents.
Printer
Thread 1
Thread 2
Thread 3
Thread 4
Fig 8.1 demonstrates the idea for this example. one without synchronizing and other with synchronizing the threads and compare the results.out.sleep(1000). This is what we call as synchronizing the threads to share the resource. Fig 8. } catch (Exception e) { } System.Chapter 8
Threads
Multi-Threading in Java
from breaking is to make the kids (threads) get hold of the toy(resource) one after the other.println("Printing " + data + " started").out. try { // Sleep for 1 second (Simulating the printing process) Thread.

It first created the Printer resource object and then created four threads passing the same resource (p) with different data as shown below:
PrinterThread PrinterThread PrinterThread PrinterThread p1 p2 p3 p4 = = = = new new new new PrinterThread(p. PrinterThread(p.
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. It didn’t follow our expected pattern. "Document3"). "Document1").Chapter 8
Threads
Synchronization
}
}
Now. look at the test class.
Synchronization
Thread synchronization is the art of making the threads access the resource one after the other without messing up the resource. PrinterThread(p. who does this synchronization? Will the resource does this or will the threads do it. "Document4"). completed. completed. Let’s see how we do this. If all the threads behaved well. PrinterThread(p. "Document2"). The question is.
Printing Printing Printing Printing Printing Printing Printing Printing Document1 Document4 Document2 Document3 Document2 Document4 Document1 Document3 started started started started completed completed completed completed
As you can see from the above.java C:/JavaTraining>java threads. You know what. both can do it. completed and so on…. then we expect the output to be in the pattern shown below: Started. In Java. started.
Finally it started all the threads. started. there is a keyword named synchronized that we can use to implement synchronization. This is what happens when all the threads simultaneously act on a single resource. To prevent this from happening we need to synchronize the printer resource.
C:/JavaTraining>javac threads\*. Let’s compile and execute the above test code with the following commands and cerify if the result indeed follows the above pattern or not.PrinterTest
The result of the above program will be something like. printing the documents is messed up.

This is where synchronized block comes in handy. right? See how simple it is for the resource to get the threads in control. Let’s say the Printer class is obtained from a third party company who didn’t synchronize the method and at the same time didn’t give us the permission to access to the source code. right? Does this mean our poor printer would be messed up by the four threads? Not really if the threads behave properly like good threads. What would you do in such situation? You cannot add synchronized keyword to the print() method.
public synchronized void print(String data){ ----------------}
Modify our print() method adding the above keyword and run the same test program again. This time to your surprise.
Synchronized blocks
A synchronized block is normally used by a thread to lock the resource from other threads before using it. Let’s see what this is. synchronized (object to lock){ } In our example.Chapter 8
Threads
Synchronization
If the resource itself want to implement the synchronization. all it need to do is declare its shared methods (print method) using the synchronized keyword. Now update the run() method in the PrintThread class as shown below:
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. let’s again remove the synchronized keyword from the print() method. In such cases. so we declare the method as. The syntax for this block is. the threads themselves need to take the responsibility to synchronize the Printer object in their own run() method. the output will be
Printing Printing Printing Printing Printing Printing Printing Printing Document2 Document2 Document1 Document1 Document4 Document4 Document3 Document3 started completed started completed started completed started completed
Now the output looks great. In our example. the resource is the print() method in the Printer class.

we use synchronized keyword.
Summary
Multi-threading is a technique in which multiple threads in a single process are executed simultaneously. which we seldom use in real world applications.print(). This is called as external synchronizing. Java uses Thread class and Runnable interface to create threads. we first synchronized the printer object “p” and then invoked the method. To synchronize the threads with the resource. and the result will still be the same good result..Chapter 8
Threads
Summary
public void run(){ synchronized ( p ){ p. Run the same program again with the above updated code. let’s summarize what we learned this far in threads. This is like a thread locking the printer object and putting the key in its pocket until it is done with the object. There are however. Which of the following interface is used to create a thread? a) Thread b) Runnable
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. } }
As you can see from the above code. Java supports multi-threading using the threads API. This completes all the important things you need to know to work with threads. Read the interview questions in the later section. All the statements that should be executed simultaneously must be placed inside the run() method of the thread class. Synchronization is the process by which multiple threads can access the resource one after the other without corrupting the resource. It can later come back to CPU and resume its execution cycle. For now. and you’ll be good to go. A thread can voluntarily relinquish control of CPU by yielding.
Time to play 50-50
1. A thread goes through several states before it finally dies. some concepts like joining the threads etc.

Chapter 8
Threads
Interview Questions
2.
R1
R2
Locked Waiting T1
Locked
T2
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. which of the following state does it go immediately? a) Running b) Runnable 4. 5. Which of the following method should be implemented to create a thread? a) public void run() b) void run(). Which of the following keyword is used to sync the threads with the resource? a) synchronize b) synchronized
Interview Questions
Question: Explain the lifecycle of a thread. See the following figure. and if T1 is waiting for R2 and T2 is waiting for R1. Answer: Refer to page Question: What is deadlocking? Answer: When one thread T1 locked resource R1 and another thread T2 locked resource R2. Which of the following class is used to create a thread? a) Thread b) Runnable 3. When a thread is started. then the condition is a deadlock condition.

C:/JavaTraining/Chapter9/misc misc is a package in which we will store the programs. To compile the programs.
C:/JavaTraining/Chapter9
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. move
to the following root directory and get ready.
Chapter Goals
Learn how to create JAR files Learn how to use JAR files Understand CLASSPATH variable Learn to use any Java API
Environment Setup
All the programs in this chapter should be stored in the following directory. you will become familiar about creating and using JAR files.Chapter 9
JAR Files and Using API
Chapter Goals
Chapter 9
JAR Files and Using API
By the end of this last chapter. understand how to use API’s and how to work with CLASSPATH.

we pack all the files into one or more JAR files and then distribute the JAR files to who ever need it. we need to know how to create JAR files. Sweet! There are two things we need to know here. and once we get these JAR files.Chapter 9
JAR Files and Using API
Introduction
Introduction
As I said at the very beginning of the chapter. and as a receiver we need to know how to use the JAR files in our application. there are several components you need to build.jar” extension. This is a standard compression format invented by the creators of Java to archive Java related files. The JAR file uses “. we can use the classes in the JAR files as if they are our own classes. like FTP application to transfer files during chatting. voice transfer application for voice chatting. Though porting a Java application is simply copying the class files from one system to another. The primary reason for the success of Java is the usability factor. Instead.
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. Developing all these components is time consuming task if we build all from scratch. Is’nt this cool? This is the best part of Java. you want to build chatting application in Java. Therefore.
Notion of JAR file
JAR stands for Java ARchive. So. As a sender. Say.zip” extension. A single Java application can use any third party Java applications with ease. We need to use some utility tools that aid in porting the code conveniently and this is what we will learn in this chapter. a Java application is nothing but a collection of several classes. Usually Java applications comprises several class files. To build this application. let’s learn both. it’s not as easy as it looks. All we need to do is use the third-party code and integrate it into our application. image transfer application for video chatting and so forth. and it’s tedious to distribute each and every file to the clients. Let me clarify this. a Jar file is nothing but a compressed file that compresses all the given Java related files into a single file. These companies usually distribute the Java code in the form of JAR files. This is just like we compress the files using ZIP tool that has “. What if there are some companies that distribute free and reliable Java code for the above components? This way we don’t have to write all the complex code using complex algorithms. We also know that Java is platform independent because we can port the class files to any operating system.

java
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.
public void stopWebCam() { System.1b (AudioTransferTool) A simple Java class
package misc.println("Audio Streaming Stopped").println("Visual Streaming Stopped"). } }
Listing 9.
The above two classes are pretty straightforward. Copy the AudioTransferTool. Following are the steps to create a Jar file. Listing 9.
2.
1. public class VideoTransferTool { public void startWebCam() { } System.class”
will now be in this directory.println(" Streaming Audio for Voice Chating").
C:/>JavaTraining/Chapter9> javac misc\*. Let’s first write couple of classes and then create a JAR file with the two classes shown in listing 9. Compile the classes using the following command.java and VideoTransferTool. public class AudioTransferTool { public void streamAudio() { System.java” and “.1a (VideoTransferTool) A simple Java class
package misc.println(" Streaming Images for Visual Chatting").out. All the “. Let’s don the role of sender and create a Jar file with the above two classes in it.1a&b.Chapter 9
JAR Files and Using API
Notion of JAR file
Creating a JAR file
A Jar file is created using the jar tool that comes along with the Java software installation.out.out.java into
the misc directory. } public void stopAudio() { } } System.out.

In such cases.class(in = 540) (out= 333)(deflated 38%) adding: misc/VideoTransferTool.jar misc\*.java(in = 263) (out= 144)(deflated 45%) adding: misc/VideoTransferTool. Assuming that we received the above audiovideo.class” files.Chapter 9
JAR Files and Using API
Notion of JAR file
3. then we tweak the above command as shown below:
C:\JavaTraining>jar cvf audiovideo.class added manifest adding: misc/AudioTransferTool. If you only want to give the class files. This is how they maintain the ownership of the code.jar misc utilities com src
This is all you need to know about creating JAR files.java” files “. all we need to do is simply list all the directories we want to archive with a space delimiter as shown below:
C:\JavaTraining>jar cvf audiovideo.class(in = 545) (out= 342)(deflated 37%)
The above command just added the “. Most people would only archive the class files only. it added both “. Copy the above Jar file to the following directory:
C:\Jars
Using a JAR file
Using the Jar files is the best part of Java. classes will be distributed in more then one package. Finally. let’s write a chatting program that uses the two classes in the jar file. to create a Jar file with classes in different packages.class(in = 540) (out= 333)(deflated 38%) adding: misc/AudioTransferTool.class” files.jar to include the source files
and class files that are in misc package using the following command:
C:\JavaTraining>jar cvf audiovideo. If you noticed the above verbosity.jar misc added manifest adding: misc/(in = 0) (out= 0)(stored 0%) adding: misc/AudioTransferTool.
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.class(in = 545) (out= 342)(deflated 37%) adding: misc/VideoTransferTool. create the Jar file named audiovideo.jar created in the same directory. Many a times. They will never distribute the source code.jar file via email or some other means.java(in = 269) (out= 153)(deflated 43%)
The Jar command displays all the verbose of what it did on the console. You’ll now see audiovideo.

2a (ChatApplication. Therefore.out.streamAudio(). atool.out. and invoked the methods as shown below:
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.println("Application Ready for chatting"). Therefore.java
Look at the following import statement:
import misc. The above statement imports all the class files in the misc package.println("Application finished chatting").
Since the classes in the jar file are in the misc package. System. we are ready to use the classes in the Jar file like our own classes. atool. Listing 9.println(" Using the Audio Tool"). right? Once we finished importing.java) A simple chat application class
package chatapplication.
C:\JavaTraining\chapter9\chatapplication\ChatApplication. vtool. } } System.stopAudio().out. this program should to be saved in the chatapplication folder as.Chapter 9
JAR Files and Using API
Notion of JAR file
Look at the code in listing 9.out. System.*. AudioTransferTool atool = new AudioTransferTool().println(" Chat application terminated"). System. vtool. public class ChatApplication { public static void main(String args[]) { System.startWebCam().stopWebCam(). System. we created objects of the classes.*. which is what we want.2. import misc. we need to import them using the import statement. we created the above class in the chatapplication package.
For convenience purposes. VideoTransferTool vtool = new VideoTransferTool().out.out.println("Using the Video Tool").println(" Initializing Chat Application ").

^ chatapplication\ChatApplication.
C:\JavaTraining>chapter9>javac chatapplication\*.Chapter 9
JAR Files and Using API
Notion of JAR file
AudioTransferTool atool = new AudioTransferTool().streamAudio(). atool.jar file. ^ chatapplication\ChatApplication.ChatApplication AudioTransferTool atool = new AudioTransferTool().ChatApplication VideoTransferTool vtool = new VideoTransferTool(). It says it cannot resolve the imports for Audio/Video classes. ^ 5 errors
Shoot. The default
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.java:12: cannot resolve symbol symbol : class AudioTransferTool location: class chatapplication. No wonder why the compiler bombed at us. The compiler is not happy.java:17: cannot resolve symbol symbol : class VideoTransferTool location: class chatapplication.
Let’s compile the above class with the following command and see what happens. This is where something called CLASSPATH variable comes into picture.ChatApplication AudioTransferTool atool = new AudioTransferTool(). From program point of view. We know that the above classes are in the audiovideo. ^ chatapplication\ChatApplication.java:12: cannot resolve symbol symbol : class AudioTransferTool location: class chatapplication. How does the Java compiler know where these classes are? This is like asking the compiler to compile a program that uses third party classes without telling it where the third party classes are.ChatApplication VideoTransferTool vtool = new VideoTransferTool(). We wrote the program that uses the classes in the Jar file.*.java:3: package misc does not exist import misc. we did everything right. we did.java:17: cannot resolve symbol symbol : class VideoTransferTool location: class chatapplication. During the installation of Java software. Let’s first understand what the CLASSPATH variable is before we fix the above errors. but how do we tell this to the compiler? Good question.java chatapplication\ChatApplication. Did we miss anything here? Yes. a default value for this CLASSPATH variable will be set.
CLASSPATH
CLASSPATH is a standard variable that Java compiler uses to determine the location of classes that are to be loaded before compiling the program. This is just not sufficient. ^ chatapplication\ChatApplication.

Goto Start->Settings->Control Panel-> System->Advanced->Environment Variables.jar
The problem with the above command is that. Select the CLASSPATH and click Edit button. It will display the following window:
2.c:\jars\audiovideo. There is a way to get rid of this problem using the following process: 1. This will display a small window as shown below:
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. we need to append the location of the new Jar file to the existing classpath as shown below:
C:\JavaTraining>chaper9>set CLASSPATH = %CLASSPATH.Chapter 9
JAR Files and Using API
Notion of JAR file
value is nothing but the location of the built-in library classes. To specify the location of third-party Jar files. then you again have to type the above command before executing the program. if you close the command window.

This is all you need to know about creating Jars and using Jars. You can then execute the program like we did before to see the following output.
C:\JavaTraining>chapter9>java chatapplication. You just have to remember one thing. In real world applications. and all the errors will disappear. Keep Clicking OK until all the windows disapprears.********
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. there will be fancy editors where you simply click the easy button to do all the above tasks.Chapter 9
JAR Files and Using API
Notion of JAR file
3.jar
When the Java compiler compiles the program.c:\Jars\audiovideo. it will then compile the program without any errors. Now. Since the location of the audiovideo. Append the following at the end as shown and click OK. I am sure the above commands are simple and pretty straight forward.jar is also specified in the class path. it first checks the above CLASSPATH variable and then compiles the program.ChatApplication Initializing Chat Application Using the Audio Tool Streaming Audio for Voice Chating Using the Video Tool Streaming Images for Visual Chatting Application Ready for chatting Application finished chatting Audio Streaming Stopped Visual Streaming Stopped Chat application terminated
If you have more than one Jar file.
. compile the chat application program again. *****Put the Jar files in the CLASSPATH before using them. you have to append all the JAR files to the CLASSPATH variable. Cool.

then we cannot use their technology and they are the ones who lose their business. The reason is simple. they also give you the Java documentation for all the classes used by the technology. “Let’s learn Servlet API”. all this documentation is available online. Usually any company that develops a technology using Java. If you need the API for any Java technology. and it will pull it up for you. So. If someone like your manager comes to you and say. “Use Spring API to build database applications”. trust me. As I said before. API stands for “Application Programming Interface”. if I say. We then use the API to understand the functionality of the classes. Use the Jar files that Spring has provided to build the application and 2. This is a funky name given to the Java documentation. 1. You can also download the documentation if you want.
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. The good thing about Java is that. and then use them in our programs. If they don’t give you the documentation. Since it’s virtually impossible to remember the names of each and every class. This is nothing but the Java documentation that we saw in the previous chapter. just type “Struts API” it in Google. So. provide us with the so called API which is nothing but Java documentation in HTML format that we saw in the previous chapter. Use the API (HTML documentation) to learn about the classes in the Jar files. When you download a Java technology from a company. what he means is. for instance Struts technology. The API will look as shown in the following figure. use the documentation and understand the details about the servlet classes. what I mean is. there must be documentation about all the classes that we can use to learn the functionality the classes offer. there will always be documentation for any Java technology. Like wise. don’t panic to learn all the classes. a Java application is a collection of several classes and interfaces.Chapter 9
JAR Files and Using API
Understanding API
Understanding API
This is the buzzword that you always hear with any Java application.

simple rules to explore any Java documentation 1. hearty congratulations and a job well done. it will display all the class names (left bottom). If it has static. use ClassName. To use a particular method. 3. Take my high five.Chapter 9
JAR Files and Using API
Understanding API
If you look at the above figure. You can click on the links to know about each class. Finally. Open the Class by clicking on the hyperlink. 4. then create the object using one its constructors listed on the same page.methodName syntax to invoke the method. check if the method has static keyword or not. If it is non-static (object method). So. and all the packages it has (right side). pass all the parameters and take whatever it returns. Again. Pass what ever parameters it needs. and take what ever it returns. and then invoke the method using the object reference.
Now is the moment to call yourself as an expert Java Programmmer.
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. 2.

Suggestion
Most people are scared of Java because of the number of technologies it has and remembering the class names in these technologies. Trust me. J2EE is lot easier than Java fundamentals.
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. I am going to teach you something called J2EE. When you come back. Very important. you don’t have to memorize all the classes as long as you know what the program is doing. Always make sure that you move forward and make sure you don’t loose your heart. You’ll be fine. But you know what. then forget about the technology. Take my word. skip it temporarily and revisit it later. take a small break and enjoy yourself. Keep the ball rolling.Chapter 9
JAR Files and Using API
Suggestion
Now that you have become a very good Java Programmer. If some technology or even a program frustrates you.
Congratulations on successfully completing the Core Java fundamentals. When you revisited and if it still frustrates you.

Chapter 9
JAR Files and Using API
Suggestion
Part
2
J EE
254
.

Chapter 10
Introduction to J2EE
Chapter Goals
Chapter 10
Introduction to J2EE
This chapter introduces you to the world of J2EE by highlighting the important J2EE technologies and their application. Understand the core J2EE technologies supported by the application servers Know the list of free and commercial servers available in the market Understand the various J2EE architectures.
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.
Environment Setup
Not Needed.
Chapter Goals
Understand the notion of J2EE Understand the key challenges during enterprise application development Understand the key challenges when an application is run on internet Understand what a J2EE application server is. This will also give an idea about J2EE application servers and their role in building enterprise applications.

we will use Java to build enterprise level applications that run on internet. So. As I told you at the beginning itself. businesses quickly realized a whole new market had opened up to exploit and started taking advantage through e-commerce. standalone and internet based applications. With almost all the businesses going online. All the Java programs we learned and developed until now are standalone applications. as they form the faces of today’s modern enterprises on the internet. This is one side of Java that we all know about. You can take my word on this. This is the true power of Java. C++ etc. Java is used to build two types of applications. These applications are like independent applications that are used by several users and every user will have a copy of the application. Ecommerce applications have now become the global faces of business companies and completely changed the dimensions of businesses. With the widespread usage of internet. paint tools etc where funky windows show up to display information. adoption of sophisticated technologies has become the key factor in exploiting the information assets of a business. we finally landed in the world of J2EE.
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. or GUI based applications like word processors. let’s look at the key challenges that every enterprise application faces. The advent of internet has completely changed the world of communication. J2EE is one such technology that helps businesses to build better and flexible e-commerse applications by securing the critical business data. except being platform independent. An enterprise or e-commerce application is nothing but an application that an enterprise or an organization uses to do business using Internet.Chapter 10
Introduction to J2EE
Introduction
Introduction
Having learned all the good things in Java. from now onwards. The other side of Java is that it can be used build Internet based applications that can be used by millions of users at the same time. Let me tell you one thing. E-commerce is nothing but doing business using internet and has become an instant success. In order to sustain the competition. Now that we know what an enterprise application is. These applications are either console based applications that spit the output to the console. Such Java applications are no different from applications built using other languages like C. Such internet applications are popularly known as enterprise level applications. the information assets of a company became more and more valuable. J2EE is unbelievably easy and simple.

We see two important challenges here. Challenge 2: We are trying to build enterprise applications in Java. what format the request should be sent. Security: The application must be able to provide a secure environment during the exchange of information between customers and businesses. C++ etc. All the above are the challenges that an enterprise application faces after it is completely developed. Availability: It’s very important that the application be up and running all the times with almost zero downtime. So. and all we need to do is use such technologies to talk with them. we need someone to develop these technologies in Java itself. Therefore. If you own a business. what kind of response we get back and all that good stuff. we need some ready made specialized helper technologies that already know how to interact with such complicated systems. messaging systems and what not. Challenge 1: Making our Java program interact with such complex systems. to make our life easy. its very important that it responds faster enough to user requests. mainframe systems. you want to spend more time writing code that serves your customers rather then writing code to connect with databases. You agree with me? Good. right?
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.? Again it gets complicated (but not impossible) for our Java program to use the technologies. legacy systems etc.Chapter 10
Introduction to J2EE
Introduction
Performance: Since the application is now exposed to millions of users. Then a Java program using Java technologies will be pretty straight forward. What if the above mentioned ready made helper technologies are written in some other language say C. The result is "Your company is losing serious business". there are few challenges while developing the application itself. Few seconds of downtime can result in losing thousands of $$. It is very important for us to understand the details of the system that we want to interact with like say. what protocol to use to send the request. However. This is one of the most important aspects that play a key role in the success of a business. Reliability: The application must be reliable in terms of processing the business transactions successfully and accurately. Building enterprise application is not that simple as it involves interactions with several software systems like databases. you can't serve your customers without connecting to such systems. right? But again. Understanding such systems and writing code to interact with such systems is a time consuming task and delays building enterprise applications.

Address the challenges (Performance. So. We now came to a point. thus promoting rapid application development 2.1 Core J2EE Technologies Technology Name JDBC Servlets/JSP JNDI JMS EJB Javamail Description Used for easy interaction with various Databases Used for building dynamic web applications in Java Used for interacting with Naming and Directory Services Used for interacting with Messaging Systems Component technology for building distributed components Used for sending Emails
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. configuring the technologies. Learn to build enterprise applications using the above technologies. Table 10. Provide a runtime to run or host the applications on the internet 3. Learning J2EE is nothing but. where we need a sophisticated infrastructure that 1. managing such technologies is yet another challenge. J2EE technologies are standard helper technologies that we use to build enterprise applications.
Core J2EE Technologies
As I said before. This is exactly what we will do from here onwards. 1. Availability etc) when the application runs on the internet J2EE provides one such sophisticated environment which we call it as J2EE Application Server. running our enterprise application that uses the technologies.Chapter 10
Introduction to J2EE
Core J2EE Technologies
Challenge 3: Assuming that we have all the ready made helper technologies written in Java. we need some sort of sophisticated environment which we also call as runtime that helps in addressing this challenge. managing the lifecycle of the technologies and above all. Addresses the challenges during application development through ready made technologies. For instance.1 shows the list of important technologies that every J2EE application server supports: Table 10. Learning the helper technologies that it comes with and 2.

are very popular and demanding in the world of J2EE. Instead. These technologies have become very popular and almost every real world J2EE application since the last couple of years is using them. This is why we call them as open source technologies. Table 10. However. companies will also distribute the source code (Java source files) along with the class files. ok. Following is the list of popular open-source Java technologies. You know what. In simple terms. An enterprise level application is a very complex and huge application. there are several open-source technologies (free of cost) in the market though not adapted as standard technology in J2EE platform.1. To understand this. It is impossible for a single person to build it from scratch.
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.1. there will several teams to build the entire application. you really don’t have to know all the technologies. Open Source technology is free to use and a standard J2EE technology is proprietary.2 Open Source J2EE Technologies Technology Name Description Struts Used for building dynamic web applications Spring Container framework for building J2EE applications Hibernate Framework for building Persistent Java applications Question: How is an open-source Java technology different from standard Java technology? Good question. take a look at Fig 10. and every team is specialized in a particular area. With open-source technology. so that you can modify and use if needed. Companies will never give the source code.Chapter 10
Introduction to J2EE
Core J2EE Technologies
The above listed technologies are standard J2EE technologies that are supported by every application server. But this is not the case with standard J2EE technologies listed in Table 10. I am sure you are disappointed to know that you have to learn all the above technologies in the two tables.

So. The technologies used by the teams are listed under the team names. then you are good if you know just these two technologies. if you happen to get a Job as Java Web Developer. JNDI etc JMS. However you can also work in all the teams at the same time. JCA etc
Fig 10.1 Enterprise Application development Teams and Technologies
If you look at the above figure. we see like three different teams involved in the entire application development.Chapter 10
Introduction to J2EE
Core J2EE Technologies
J2EE Enterprise Application
Teams
Web Development Team Middle Ware Team Integration Team
Technologies
Servlet/JSP Struts Spring. All the teams together are called as Server Side developers which means if you are get a job as server side developer. But it’s always good to know the purpose of each and every technology so that you are not lost during discussions with other teams. All this is to say that you don’t have to worry about the number of technologies you have to learn. The web development team uses Servlet/JSP and Struts technologies.
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. you might work in any of the above teams based on your specialization and expertise. JDBC EJB.

You need to strictly follow a systematic process to make the application running and working. What is it used for. This is because J2EE is process oriented. We will only learn those technologies that are widely used in real world applications.
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. but the real challenge is getting it running. I never worried even if the program failed to work as long as I know that I followed a systematic process and what I did makes sense. If you still don't get something to work. you need to know the following: 1. If you can get the application working. That's how I learned J2EE. Used for building dynamic web applications in Java Container framework for building J2EE applications Framework for building Persistent Java applications
EJB technology is an ocean and the complete details of it are beyond the scope of the book. Writing J2EE programs is very simple. This challenge can be addressed by keeping things simple and following a systematic approach which is exactly what we’ll do in this book. It hardly takes few minutes to write the program. At this point let me tell you one thing. At the bare minimum. I am sure you'll enjoy it. right. Most of the real world applications are slowly trying to get away from EJB due its complexity and moving to Spring/Hibernate which will be covered in detail. The way to learn J2EE is. Learn J2EE with passion and more than that with patience.Chapter 10
Introduction to J2EE
Core J2EE Technologies
This book covers in detail the following latest and greatest technologies that are used in today’s real world J2EE applications. learn to build applications using technologies. Table 10. Fair enough. What is the name of the technology 2. don't loose your heart. The whole purpose of this book is teaching you Java and J2EE that is needed to land in a good job. As long as you know what you are trying to accomplish. it’s a BONUS. Trust me.2 Important J2EE Technologies Technology Name JDBC JMS Servlets/JSP Struts Spring Hibernate Description Used for easy interaction with various Databases Used to talk with messaging systems. knowing these is more than enough to land in a good job in J2EE world. you are good. I will outline all the steps that you need to follow to run the applications.

The only technologies used to build web applications are: Servlets/JSP and Struts We can call the above two technologies as mainstream technologies. You only need a server primararily when you build Web applications. Therefore. Hibernate we will run them as standalone applications using javac and java commands. we need to know the architecture of a typical enterprise application. Spring.2 Typical 2-tier architecture
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.
Enterprise Application Architectures
Since we are almost ready to start building enterprise applications in Java.2 that shows typical 2-tier architecture. Therefore if my application has 100 clients. Keep this in mind.Chapter 10
Introduction to J2EE
Enterprise Application Architectures
Let me ask you a question here. However the database will still reside on a separate computer that will be shared by all the clients. Before we look into it. let’s learn something about the various enterprise architectures that we currently have. when we learn helper technologies like JDBC.
2-Tier Architecture
Look at Fig 10. All others are helper technologies that the mainstream technologies use to build the applications. I need to install the application on all the 100 client computers. Do all the technologies require the use of an application server? Good question. all the business logic and presentation logic of the application is embedded in the clients computer itself.
PT Client 1 BT
PT Client 2
BT
DB
Fig 10. We will only use the server when we write applications using Servlet/JSP and Struts technologies. The answer is No. In this architecture.

The main difference is that the business logic is permanently isolated from all the client systems and moved to centralized middle tier.Chapter 10
Introduction to J2EE
Enterprise Application Architectures
Advantages Simple and easy to build Low cost Disadvantages Even a small change to the application requires a reinstall of the application on all the client computers.
3-Tier Architecture
Fig 10.
PT Client 1 BT PT Client 2 DB
Fig 10. business logic and data are logically distributed in three tiers. The disadvantages of this architecture outweigh the few advantages making it less useful.3 shows typical 3-tier architecture. The performance of the application is dependent on the performance of client computer. Heavy network traffic due to multiple requests to the database from all the client computers. In this architecture. If some clients are not ready to take the changes. thereby causing maintenance nightmares. This architecture overcomes all the cons of 2-tier system.3 Typical 3-tier architecture
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. the presentation logic. The presentation logic on the client systems will query the business logic in the middle tier which in turn accesses the data from the database. then multiple versions of the application will prevail.

4 Typical n-tier architecture
264
. A typical n-tier architecture contains the following elements: User Interface: This is something like a web browser that handles the client interactions.
Application Clients Shared Services
Databases
PT Browser BT
File System
Fire Wall
Data Tier
Fig 10. the application logic is divided based on the functionality rather than physically like in 2-tier and 3-tier architectures. Disadvantages Any update to the business logic must be accepted by all the clients even if they are not ready for updates.
N-Tier Architecture
In this type of architecture. Data tier: This contains all the enterprise data. Application performance is no longer dependent on client computer due to the business logic isolation. Less network traffic. thereby improving the performance of the application. Presentation Logic: This defines format using which the information will be displayed. No more maintenance nightmares. Business Logic: Encapsulates all the business rules by interacting with data sources.Chapter 10
Introduction to J2EE
Enterprise Application Architectures
Advantages Centralized business logic will offer more flexibility. Business logic is only required to be changed at one place there by eliminating the installation process of the application on client systems. Infrastructure Services: These are utility services that the presentation and business logic makes use of.

However. This is the one that runs the enterprise application on the internet. an enterprise application is a collection of several applications within the enterprise with all the applications working in tandem and interacting with each other through well defined interfaces a shown in the following figure.
Enterprise Architecture
An N-tier architecture is applicable to a single application in an enterprise.Tier Data Tier Fig 10.Tier
Databases
File System Client B N .
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.
Client A
N . better maintenance. let’s now try to understand somethings about application servers. So.
J2EE Application Servers
There is some terminology you need to understand when it comes to J2EE. we don’t use java command to execute the J2EE applications.Chapter 10
Introduction to J2EE
J2EE Application Servers
Breaking the application logic based on functionality offers several benefits like flexibility. However.5 Enterprise Architecture
Simply stating. An application server is the heart of any J2EE application. We use a special program called application server to run the applications. With this basic knowledge of various architectures. enterprise architecture is a combination of several n-tier architectures. we can’t even move a step forward without knowing what this is. we still use the same old javac command to compile the applications. improved performance. This architecture is also referred to as Model-View-Controller (MVC) architecture. reusability of code and may more. Unlike simple Java programs.

Therefore. To simplify the development of such applications. IBM. These are popularly known as J2EE specifications. This is how IBM. IBM. JBoss and many more.3 J2EE Application Servers Company SUN IBM BEA JBoss Server Name SunONE Studio Application Server IBM WebSphere Application Server BEA WebLogic Application Server JBoss Application Server
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. They did the hard work and came up with the specifications for the above. it finally chooses the company who sells it for less $$ and more value. Once you have a J2EE application server. It will therefore hunt for those who sell the J2EE servers and come across several companies like SUN. What is the purpose of all these companies developing the same application server? Good question. and built the two workable components listed above using their expertise. After serious negotiations with the companies. Apache and several other companies came into picture. they started selling it. Having just the specifications is not good enough. Here is the deal. BEA. BEA. Sun Microsystems first came up with the notion of J2EE platform that can be used to build large scale enterprise applications in Java. Once they built the application server along with the helper technologies. There are several free and commercial application servers available in the market today that are developed by noted companies like Sun Microsystems Inc. These companies used the J2EE specifications from Sun Microsystems. BEA and many more. We need someone to convert the specifications into workable realities. A Server that runs the applications 2. they felt the need for the following two things: 1.Chapter 10
Introduction to J2EE
J2EE Application Servers
A J2EE application server is a ready made sophisticated application that will host and run all the J2EE applications. Helper technologies to build the applications. you can start building and running enterprise applications. A company to build and host enterprise level applications in Java needs a J2EE application server. All the companies that built the J2EE Application Server gave funky names to their product shown in the following table: Table 10. Sun Microsystems decided to distribute the specifications to companies who can implement them.

We therefore need a vendor who just gives us a Web container. isn’t it? Not only that we got a web container. Look at the following figure that shows the architecture of a J2EE application server. Wow. purchasing an application server is not a good idea. and application components built using EJB technology are run within EJB container. JNDI. Remember this one point. Let me ask you one question here. A container is nothing but a runtime environment to manage the application components. “A J2EE application server runs enterprise internet applications that are built using standard J2EE technologies”.
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. Application components built using Servlet/JSP/Struts technologies run within Web container. but also for free of cost. An application server has both Web container and EJB container. You cannot run EJB components in Web container and Servlets/JSP components in an EJB container.
Web Container
EJB Container
Servlets/JSP
EJB
JDBC
JMS
JavaMail
JNDI
RMI
Fig 10. In such cases. Cool deal. Hibernate etc) are like helper technologies that are used by the above two mainstream technologies in both the containers.6 J2ee Application Server Components
A J2EE application server comprises of a Web Container and an EJB Container. JMS. All the remaining technologies (JDBC. If our application doesn’t use EJB technology. In J2EE. and you are good. This is really cool. This web container is FREE to download. having just a web container is good enough. This is where a company named Apache Software Inc. ok.Chapter 10
Introduction to J2EE
J2EE Application Servers
To summarize. there are two mainstream technologies like Servlets/JSP/Struts and EJB. Therefore. comes into picture who developed a Web container named Tomcat. Spring. We don’t need an EJB container. do we need an EJB container? You are right.

A J2EE platform is also known as J2EE Application Server.
Time to play 50-50
1. We use J2EE platform to build large scale enterprise level applications in Java that run on Internet. But for now. EJB. developing J2EE application is fun and I am sure you’ll enjoy it. JMS etc. having just the web container will solve the purpose. We don’t need an application server if we don’t use EJB’s for application development. let’s begin the journey into J2EE with passion. Until then don’t worry about it. One example of n-tier architecture is a Service Oriented Architecture (SOA). let’s start learning the J2EE technologies. Servlet. JSP. This is developed by Apache Software Foundation. Besides the standard J2EE technologies. without wasting any further time. Tomcat is a FREE web container available in the market today. A J2EE application server supports all the standard J2EE specifications like JDBC.
Summary
J2EE stands for Java 2 Enterprise Edition. In such cases. An application server is bundled with both Web Container and EJB Container. Most of the J2EE applications using n-tier architecture in which the application logic is logically separated based in the functionality. lets summarize this chapter. we’ll see how to install it. Trust me. When we get to a point where we need to use the Tomcat container.Chapter 10
Introduction to J2EE
Summary
In this book. J2EE is a platform and not a language. there are also several popular opensource technologies. Which of the following statement is true? a) J2EE is a language like Java
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. Spring and Hibernate. Application server also comes with runtime to run the applications that use standard J2EE technologies. These include Struts. The most popular application servers are IBM WebSphere and BEA Weblogic servers. With all this basic understanding of what J2EE is all about and J2EE application servers. There are several free and commercial J2EE application servers available in the market. we will use this FREE Tomcat web container to run the applications. So.

Which of the following statements is true? a) An application server comes only with Web Container b) An application server includes both Web container and EJB container 6.Chapter 10
Introduction to J2EE
Time to play 50-50
b) J2EE is a platform used to build enterprise applications using Java 2.
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. Scalability. Which of the following is a standard J2EE technology? a) Servlet/JSP b) Struts 3. as it just an introductory chapter. Which of the following is an open-source technology to build Web applications? a) Struts b) Hibernate 4. Availability. Security and Maintenance b) Usability and Presentation There will not be any interview questions in this chapter. Which of the following is the name of the application server from IBM? a) Websphere b) Weblogic 5. Which of the following architecture is widely used for building J2EE application? a) 2-tier b) n-tier 8. Which of the following are the main challenges with a J2EE application? a) Performance. Let’s move on to the next chapters to begin our journey into the world called J2EE. Which of the following is a Web Container? a) WebLogic b) Tomcat 7.

This chapter will explain the details of what a database is. This chapter also gives you a brief over of SQL and it syntax. Compiling and executing the program
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. and how to execute the SQL queries against the database. Understand the notion of Connection Pooling Understand the common problems in typical databases and solutions to address them. how to use JDBC technology to connect with databases.
Chapter Goals
Understand the basics of Databases and Drivers Understand the basics of SQL language Understand the important components in JDBC API Learn to write Java programs to connect with databases Learn to write programs to store data in the database tables Understand various database operations. 1.Chapter 11
JDBC
Chapter Goals
Chapter 11
JDBC
By the end of this chapter. you will be an expert in writing Java applications that talk with databases.
Environment Setup
Compiling and executing JDBC program requires. Placing the required Jar files in the CLASSPATH 2.

jar.bat’ and execute the batch file before we start executing the programs.3-bin.3 into the C:/JavaTraining/j2eelib directory. Create a batch file named “env. the first thing you need to do is execute the batch file to setup the classpath as shown below:
C:/JavaTraining>chapter11>env.
C:/JavaTraining/chapter11/jdbc
Note: Whenever you start working with JDBC.jar from C:\sun\AppServer\lib and mysql-connector-java5.
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. 1.jar from C:\MYSQL\drivers\mysql-connector-java-5.3-bin.bat
Once you execute the above command you are ready to compile and execute any JDBC program in this chapter..bat” as shown below
C:/JavaTraining/chapter11/env.jar 5.0. 2. C:\JavaTraining\j2eelib\j2ee. Create a directory named jdbc as shown below to store the JDBC examples. Install J2EE SDK.C:\JavaTraining\ j2eelib\mysql-connector-java-5. Follow the instructions below to complete the JDBC setup process. MySql and DbVisualizer software following the instructions in “Software Installation” chapter. Copy j2ee.bat
and copy the following contents into it set CLASSPATH=. Create a directory named j2eelib as shown below:
C:/JavaTraining/j2eelib
3. 4.0.0.Chapter 11
JDBC
Chapter Goals
Since it’s always tedious to set the classpath. we will define the classpath setting in batch file named ‘env. This makes life easy.

let’s rock.Chapter 11
JDBC
Introduction
Introduction
The success of any business depends on the information and data assets it possesses. Following figure shows how the data is stored in databases: Table: Customers Name SSN Age Country John 1234567 47 USA Smith 3435345 21 Canada
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. So. One such systems are the so called Databases. retrieving the data and many more. This feature offers more flexibility and power to retrieve the data. they are highly inefficient in many respects like the amount of data that can be stored. A single table databases doesn’t mean that they have just one table. Though files are used for storing data. In order for the businesses to be competent. The main drawback with these databases is the lack of flexibility while retrieving the data. Therefore understanding the basic details of databases is very important for building enterprise applications. manipulate and retrieve the data to fulfill user’s requests. Therefore it is very important for a business to have a system for storing and retrieving data. Most of the modern enterprises use relational databases. we will first learn the basics of databases and then learn how to use JDBC helper technology to access the databases from Java programs. In this chapter. Single table databases and 2. Any enterprise application usually queries the databases to store. On the other hand in multi table databases. Databases typically form the back bone for any e-business company. Databases store the data in the form of tables with each table containing several records of data. Multi table or relational databases. These databases are therefore called as relational databases. they need sophisticated systems for storing complex and heavy volume of data and at the same time retrieve the data faster enough to process the data. two or more tables are related with other.
Database Basics
A database as the name suggests is a sophisticated system for storing the data. Databases are broadly categorized into two types as listed below: 1. It will have several tables of data and all the tables are independent of each other.

they are related with each other by the common column SSN. A typical database will have ‘n’ number tables with each having ‘n’ number of records.Chapter 11
JDBC
Structured Query Language (SQL)
The above table shows customers table that has two records of data. To pull the customer information.
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. manipulating the data in the tables.This is where SQL comes into picture. Let’s quickly learn the usage of SQL to do various database operations. deleting the data in the tables and the tables itself. So. let’s see what this is all about.
Database
Now that we know what relational databases are capable of. SQL is used for creating the tables. we need to know something about how to store the data and retrieve the data to and from databases. we need to pull the data from both the tables based on the matching SSN. let’s use the following figure to represent a database. The two tables together represent the customer information.
Structured Query Language (SQL)
Structured Query Language which is abbreviated as SQL is a standard query language for working with relational databases. For here onwards. A relational database can store the data in several tables that are related with each other as shown below: Table: Customers Name SSN Age Country John 1234567 47 USA Smith 3435345 21 Canada Table: Addresses SSN AddressLine 1 AddressLine 2 City State 1234567 Apt # 20 1111 S St LA CA 3435345 Apt #2345 2222 N St Detroit MI If you notice the above tables. populating the tables with data.

city. COLUMN 2 NAME COLUMN 2 TYPE. there are several other data types.1 Important SQL data types SQL Data Type VARCHAR NUMERIC DATE Description Used for storing text data Used for storing numbers Used for storing dates
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.Chapter 11
JDBC
Structured Query Language (SQL)
Creating a table
Let’s create a table named Customers to store customer data like firstname. COLUMN N NAME COLUMN N TYPE ). lastname. SSN NUMERIC(10). .
While creating table. state. age. . the SQL query will be as shown bellow:
CREATE TABLE Customers ( FirstName VARCHAR(50). Age NUMERIC(10). and country as shown below: Table : Customers FirstName LastName SSN Age City State Country
The SQL syntax for creating a table is shown below:
CREATE TABLE <TABLE NAME> ( COLUMN 1 NAME COLUMN 1 TYPE.
Using the above syntax. it’s very important to mention the type of data that will be stored in various columns. ssn. State VARCHAR (2). Likewise. Table 11. City VARCHAR (32). to create the customers table. The numbers in parenthesis represent the maximum length of data that can be stored.1 lists some of the most important SQL data types: Table 11. LastName VARCHAR (50). NUMERIC is used for storing numbers. VARCHAR is a standard datatype for storing text data. Country VARCHAR (32) ).

COLUMN 2 NAME = NEW COLUMN 2 VALUE … WHERE COLUMN 1 NAME = VALUE 1 AND COLUMN 2 NAME = VALUE 2 ……….
Let’s say we want to change the last name to ‘Roberts’ and first name to ‘Joe’ whose SSN is 123456. ‘CA’.
Using the above syntax. 20. null. COL2 DATA …. The syntax for this query is shown below:
UPDATE <TABLE NAME> SET COLUMN 1 NAME = NEW COLUMN 1 VALUE. Numeric data must not be enclosed in single quotes. 20. ‘CA’. A single insert query inserts one record or row of data. 123456. null. ‘John’. 3. If you don’t know what value to insert in a particular column. Following is the sql syntax..
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. ‘Vegas’. 123456. you must specify a NULL as shown below:
INSERT INTO Customers VALUES (‘Smith’. we use the INSERT query.
INSERT INTO <TABLE NAME> VALUES (COL1 DATA. The update query will be.
The above SQL inserts the data in the Customers table as shown below: Table : Customers FirstName LastName SSN Age City State Country Smith John 123456 20 Vegas CA USA While using the INSERT sql. 1. All the text data must be enclosed in single quotes.
Updating records in table
To update the data in an existing record.Chapter 11
JDBC
Structured Query Language (SQL)
Inserting the data into the table
To insert the data into the tables. ‘USA’). we insert the record into customers table as
INSERT INTO Customers VALUES (‘Smith’. 2. we use the UPDATE query statement. Every data must be separated by a comma. COL (N) DATA). ‘USA’).

Case 2: Select all the customers who belong to the state of CA.
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. Since there is no WHERE clause in the query. The contents of the table would be as shown below: Table : Customers FirstName LastName SSN Age City State Country Joe Roberts 123456 20 Vegas CA USA
Retrieving records from table
Retrieving the records is the most important database operation. COL2 NAME …… FROM <TABLE NAME> WHERE <CONDITION>
There are several ways you can use the SELECT query.Chapter 11
JDBC
Structured Query Language (SQL)
UPDATE CUSTOMERS SET FirstName = ‘Joe’. then it retrieves the selected columns from all the records in the table. let’s see few cases here: Case 1: Retrieve all the customers data
SELECT * from Customers. it will pull up all the records. The result of the above query is the entire table. consider the following table: Table : Customers FirstName LastName SSN Age City State Country Joe Roberts 123456 20 Vegas CA USA Sam Perry 34534543 22 LA CA USA Serra Bates 567657 67 SFO CA USA Serry Sanders 123213 33 DEN CO USA Smith Murray 89789 45 NEV NV USA Slater Mike 676576 32 COL OH USA If the query has no WHERE clause.
In the above sql asterisk(*) means all columns of data. Based on this. The syntax for this query is as shown below:
SELECT COL 1 NAME. To better understand. LastName = ‘Roberts’ WHERE SSN = 123456
The above query will update all the customers firstname and lastname whose SSN equals to 123456. We use the SELECT query to retrieve the records.

Let’s take a look at a simple example.Chapter 11
JDBC
Structured Query Language (SQL)
SELECT * FROM Customers WHERE State = ‘CA’. in real world applications data is required to be pulled from multiple tables. lastName from Customers WHERE State=’CA’ AND Country=’USA’. The SQL language uses very general terminology. Reading the SQL itself will tell us what it is doing. Case 4: Let’s say we have the following two tables.
The above query will retrieve those records whose state is CA as shown below Table: Customers FirstName LastName SSN Age City State Country Joe Roberts 123456 20 Vegas CA USA Sam Perry 34534543 22 LA CA USA Serra Bates 567657 67 SFO CA USA Case 3: Retrieve the first names and last names whose country is USA and state is CA (multiple conditions)
SELECT firstName. Table 1: Books Publisher ISBN Title Wrox AB3456 Perl Scripting McGraw YZ6789 Core Java Table 2: Authors Author ISBN Country Smith AB3456 Germany John YZ6789 Australia
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. However. All the above select queries retrieve the data from just ONE table only (Customers).
The above sql displays the following results: Table: Customers FirstName LastName Joe Roberts Sam Perry Serra Bates This is how we need to use select query.

Chapter 11
JDBC
JDBC
If you noticed the above two (Books and Authors) tables. we are now ready to learn JDBC technology to access databases. title and country (data in both the tables) for a particular ISBN. all the columns in books table must be prefixed with “t1. With this basic knowledge of databases and SQL. t2. they are joined together using the ISBN column.Publisher. look at the where clause.Publisher. Interact with relational databases 2. t1. and t2 is the alias name for Authors table.Title.ISBN = t2.
Whenever we need to pull data from multiple tables.Author.
JDBC
JDBC stands for Java Database Connectivity which is a technology that allows Java applications to. The syntax for this query is shown below:
DELETE FROM <TABLE NAME> WHERE <CONDITION>
For instance. the query will be. Since we need to pull the data if the ISBN in both the tables match.” as shown below:
SELECT t1.ISBN = t2.Author. I am sure things are pretty simple.
DELETE FROM Customers where State=’CA’. author. the where clause will be as shown below:
WHERE t1. If we need to pull the publisher. Execute SQL queries against the databases
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.Country from Books t1. t2. to delete all the customers whose state is CA. t2.ISBN.Country
Now. In the above query. Authors t2 WHERE t1. we do so as shown below:
SELECT t1.
The above SQL deletes 3 records in the customers table. t1 is the alias name for Books table.
Deleting the records
To delete the records in the table we use DELETE query. These are all the basic SQL operations you need to know to work with any database.” and columns in authors table must be prefixed with “t2.Title. we need to use alias names for tables. t2. t1. 1. Let’s rock and roll.ISBN. Therefore.

it must use the driver of that database. A driver is like a gateway to the database. Therefore. These could be developed in any language like C. look at the following figure:
DB2
J D B C A P I
MySQL
Oracle
Java programs
Fig 11. Microsoft and many more.
Driver
Database
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.1 JDBC and Databases
The above figure should tell you the complete story. Every database will expose something called Driver through with one can interact with database for SQL operations. Note this point. C++ etc by these vendors.
How JDBC talks with Databases
Databases are proprietary applications that are created by noted companies like Oracle.Chapter 11
JDBC
How JDBC talks with Databases
To better understand JDBC technology. for any Java application to work with databases. Java applications simply use JDBC API to interact with any database. Following figure shows the relationship between driver and database.

Fig 11. the challenge is how to use it to talk with database.
Partly Java and Partly Native Driver
280
. In simple words. Every database will have its own driver program. Given a driver program of a particular database. this driver translates the JDBC calls to ODBC calls in the ODBC layer.> ODBC -> Native It’s a 2 step process. Because of the multiple layers of indirection. The only good thing with this driver is that you can access any database with it.2 shows the configuration of this driver. The ODBC calls are then translated to the native database API calls. It’s a software program that could be written in any language. This type of driver is normally used for training purposes and never used in commercial applications. There are basically 4 different types of database drivers as described below:
JDBC-ODBC Bridge Driver
This is also called as Type-1 driver. following translations will be made. To understand this. JDBC .Chapter 11
JDBC
Database Drivers
Database Drivers
A driver is not a hardware device.
Java Application
DB
JDBC
JDBC – ODBC Bridge
ODBC
Native API
Fig 11. we need to know the different types of drivers.2 JDBC-ODBC Bridge driver
As you can notice from the above figure. with this driver for JDBC to talk with vendor specific native API. the performance of the application using this driver suffers seriously.

DB Java Application
JDBC
Java
Native
Native API
Fig 11. If you look at the Type-2 driver configuration.Partly Native driver
With this driver. This is where Type-3 driver is used.
Java Application
Java Application DB Driver Server
JDBC Driver
DB
Fig 11.3 shows the configuration of this driver.
Intermediate Database access Driver Server
This is also known as Type-3 driver. the applications using this driver perform better.3 Partly Java . However there will be situations where multiple Java programs need to access to multiple databases. It acts as a middleware for the applications to access several databases.4 that shows the configuration of this driver. Fig 11. the JDBC calls are translated to vendor specific native calls in just one step.4 Driver Server
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. we only access one database at a time. The Type-3 configuration internally may use Type-2 configuration to connect to database. See Fig 11. It completely eliminates the ODBC layer.Chapter 11
JDBC
Database Drivers
This is also called as Type-2 driver. this driver is partly built using Java and partly with vendor specific API. Because of this ODBC layer elimination. As the name suggests.

Fig 11. JDBC is Java based technology. and with the driver also written in Java. besides having a database. this driver offers the best performance.
JDBC API
JDBC technology is nothing but an API. It is a set of classes and interfaces that our Java programs use to execute the SQL queries against the database. Because this is a Java to Java interaction.Chapter 11
JDBC
JDBC API
Pure Java Drivers
These are called as Type-4 drivers. To summarize. to facilitate easy and faster access to databases from Java applications. Java programs use JDBC API. The fundamental idea behind JDBC is that. database vendors built the driver itself using Java like good friends helping each other. we need a Type-4 Java driver to work with databases. and JDBC API will in turn use the
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. Because of the widespread usage of Java. we will use MySQL database for all our applications. it can directly invoke the driver program as if it were any other Java program. In this chapter. This is the driver that all the J2EE applications use to interact with databases.5 Pure Java Driver
Interview Question: What type of JDBC driver is normally used to access databases? Answer: Type-4 driver which is also called as Pure Java driver.5 shows a Type-4 driver. Now the question is. Cool deal. we are now completely ready to write Java programs using JDBC API. we get this driver program bundled in a JAR file when we download the database. Very important point to remember.
Java Application
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Java Driver
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Fig 11. Assuming you have downloaded both MySQL database and MySQL driver with the instructions in the last chapter. where do we get this Type-4 driver? For most the databases. This is really cool as it completely eliminates the translation process.

Therefore. to work with MySQL database. 3. There is a standard process we follow to execute the SQL queries. 7. but once configured. Load the Driver Get the connection to the database using the URL Create the statements Execute the statements Process the results Close the statements Close the connection. 1. Table 11.
Let me tell you one thing here.2 lists the important JDBC classes and interfaces that we can use to build applications.2 JDBC API Class/Interface DriverManager Connection Statement PreparedStatement CallableStatement ResultSet ResultSetMetaData DatabaseMetadata SQLException Description This classes is used for managing the Drivers It’s an interface that represents the connection to the database Used to execute static SQL statements Used to execute dynamic SQL statements Used to execute the database stored procedures Interface that represents the database results Used to know the information about a table Used to know the information about the database The checked exception that all the database classes will throw. we need to configure the JDBC with all the MySQL information. This information is nothing but: 1. 5. In simple words. We’ll see how to configure JDBC later. 2.
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. Name of the MySQL driver class 2. Following lists the basic steps involved in any JDBC program. 6.Chapter 11
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driver to work with the database. for a Java program to work with the database. Table 11. URL of the database schema. 4. JDBC programming is the simplest of all. we first need to configure JDBC with the above database info and then make the Java program use the JDBC.

Listing 11. run the following command:
C:/>MySQL>bin>mysqld
The above command will start the MySQL database and listens at port 3306. // Step 2: Get the connection by passing the URL String url = "jdbc:mysql://localhost:3306/MyDB".*. System.
package jdbc. let’s write our first JDBC program.getConnection(url). using the above process and the JDBC API. import java. Plase remember the above process.
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. See code in listing 11. the first thing we need to do is import the JDBC classes using the following import statement.println(con).jdbc. This is required for all the programs that use JDBC.mysql. } catch (Exception e) { e. To do so.out. public class ConnectionTest { public static void main(String args[]) { try { // Step 1: Load the Driver Class. Connection con = DriverManager.1. Its an interview question.Chapter 11
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By following the above process.Driver").printStackTrace().forName("com. } } }
If you look at the above program.1 (ConnectionTest. we need to start the MySQL database.You can close the command window if you want. With the database up and running. we can have our Java programs successfully interact with databases. Our program simply establishes a connection to the MySQL database.java) Class establishing a connection with database.sql.
Starting the MySQL Database
Before we run any of the programs.

With the above syntax.
A JDBC driver is normally specified using fully qualified name of the driver class. In Step 1.Driver").ConnectionTest
If all goes well and a connection is established. the driver class name is com.java C:/JavaTraining>chapter11>java jdbc. For MySQL database. what is a URL? A URL is something that identifies the database.getConnection(url).Chapter 11
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java. the class name will be different. the MySQL database URL will look as
jdbc:mysql://localhost:3306/MyDB
In the above URL.jdbc. you’ll see the output something like shown below. the host name will be localhost.
To get a connection to the database. MyDB is the name of the database that we created while installing the database. To get the connection. we simply displayed it.jdbc. The second step is to get the connection to the database by specifying the URL.
Connection@jhg1234
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. we need to invoke the static getConnection() method in the DriverManager class by passing the above URL as a string. which in our case is MyDB. we just have to follow the first two steps. This method then returns a Connection object as shown below:
String url = "jdbc:mysql://localhost:3306/MyDB".mysql. You can imagine schema as a something that contains all the tables in the database.forName("com. For some other databases. Connection con = DriverManager. So. we load the JDBC driver using the following statement:
Class. Every schema will have a name.
Once we got the connection object. Its syntax as shown below:
jdbc:<database name>://<host name or IP address where mysql is installed>:<port number>/<schema Name>
If the database is running on the same computer. Save the program in the ‘jdbc’ directory and compile and execute the program as shown below:
C:/JavaTraining>chapter11>javac jdbc\*.mysql.sql.*.Driver.

out .2 and 7 are always needed.See code listed in 11. let’s write a utility class that defines these three steps.2. Listing 11. import java.Driver"). public class DBUtils { static Connection con.*. // Static block to load the driver just once static { try { // Step 1: Load the Driver Class. you are good.getConnection(url).
Please
check
your
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If something went wrong. e.2 (DBUtils.println("Unable to get connection"). steps 1. So. don’t loose your heart if the program doesn’t run. con = DriverManager.mysql. let’s write this class. } public static void closeConnection() { try {
to
load
the
Driver. As long as you understood what the program is doing.jdbc.java) Simple utility class for Steps 1.2.out.
package jdbc. System.7. Trust me. In all the JDBC programs. } catch (Exception e) { e.sql. } return con. and we can use this utility in all the programs to eliminate the redundant code. Instead of writing the same code again and again. } catch (Exception e) { System.forName("com. } } public static Connection getConnection() { // Step 2: Get the connection by passing the URL try { String url = "jdbc:mysql://localhost:3306/MyDB". As I said before.printStackTrace().println("Unable Classpath"). you would see an exception message.printStackTrace().

con.close(). } } }
If you look at the above utility class." + "STATE VARCHAR(2).getConnection()." + "COUNTRY VARCHAR(40)" + ")".3 (CreateTable.sql.java) JDBC program for creating a table. import java. This class defines two methods namely getConnection() to return the Connection object and closeConnection() method to close the connection. Connection con = DBUtils. Listing 11.*. // Get the connection using our utils. if (con != null) { // Create statement from connection Statement stmt = con." + "AGE NUMERIC(2). we'll use the above utility program to get the connection at the beginning.println("Failed to close the connection")." + "CITY VARCHAR(32)." + "SSN NUMERIC(10). we have a static block to load the driver.3. Let’s first look at the code in listing 11.createStatement().
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// Step 7: Close the connection. } catch (Exception e) { System. In all our programs from now onwards.
package jdbc. public class CreateTable { public static void main(String args[]) throws Exception { createTable()." + "MIDDLENAME VARCHAR(2). The reason we put this in the static block is that we only have to load the driver once.out. and close the connection at the end. We will use this created table for the rest of the programs." + "LASTNAME VARCHAR(50). } private static void createTable() throws Exception { // Create the DDL String sql = "CREATE TABLE CUSTOMERS" + "(" + "FIRSTNAME VARCHAR(50).
Creating Table
Our next program uses JDBC to create a table named Customers in the database.

println("Could'nt create table.println("Table created sucessfully"). } } }
As you can see from the above code.Chapter 11
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// Execute the statement by passing the sql int result = stmt. } else { System. Please check your SQL syntax"). } // Close the statements and Connections stmt. we need to create a statement (Step 3). } else { System. To execute the sql query.
Once we got the connection. we created a class with a main method.
The above method returns an int that specifies the status of query execution. For this.closeConnection(). otherwise.out. we need to invoke the createStatement() method on the con object obtained in Step 2 as shown below:
Statement stmt = con. The createTable() method first defines the SQL query for creating the table and then gets the connection using our utility class as shown below:
con = DBUtils. The main method invokes the createTable() method which is where all the JDBC logic is written.executeUpdate(sql).println("Unable to get the connection"). DBUtils. we use the executeUpdate() method and pass the sql as a string parameter shown below (Step 4):
int result = stmt. If the value of the result variable is anything other than -1.out.getConnection(). then it means the table is successfully created. it means some thing went wrong during SQL query
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.
The above method returns a statement object that we can use to execute the SQL query.executeUpdate(sql).out.close(). if (result != -1) { System.createStatement().

open the DbVisualiser tool we installed and connect to the database.
That’s it.txt as shown below:
Data.Smith.22. Read the file line by line
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.USA
Copy the above file into the following directory as.CO.Chapter 11
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execution.CO.890890.closeConnection().USA Joe. Once all this is completed.Denver. To spice up the things.33.txt
John.Denver. Note: For all the SQL operations that doesn’t return any results like CREATE.CreateTable
If all goes well you notice the following output:
Table Created Successfully.Roberts.R. we finally need to close the statements and the connection as shown below:
// Close the statements and Connections stmt.CO.USA Rob.Ball.20. You’ll see the customers table created. What we will do is.43.
C:/JavaTraining/chapter11/jdbc/Data.Denver. DBUtils.CO. DELETE.ALewis.Denver.H. 1.M.USA Sam.CO.P. Compile and execute the program as shown below:
C:/JavaTraining>chapter11>javac jdbc\*.
To verify this. UPDATE we always use the executeUpdate() method on the statement object which returns an int.30.567567.12345. INSERT.txt
The above file has five records and every record has eight columns of data separated by a comma.java C:/JavaTraining>chapter11>java jdbc.close().Denver.USA Mike. let’s assume we have all the data in a file named Data.123123.
Inserting Data
The next thing we need to do is populate the above Customers table with all the data.7686456.The int value specifies the number of records that effected.Perry.

closeConnection(). // Execute the statement by passing the sql int result = stmt. breaks the record using the StringTokenizer class.Chapter 11
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// Get the connection using our utils. and constructs a query string in proper format. Click on the Customers table. In order to retrieve the data.out. Compile and execute the code to see the following result. open the DbVisualiser tool and connect to the database.executeUpdate(sql).out.createStatement().println("Unable to get the connection").getConnection(). we use the following method on the statement object:
public ResultSet executeQuery ( String sql )
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.out. } } }
The above program though long. } else { System. } else { System. DBUtils. the main() method reads the file record by record. The code in this method is just the same as previous program. Connection con = DBUtils. All it does is.println("Unable to insert record.
To verify this.close().
Inserted 5 records successfully. Please check your SQL syntax"). It then passes the SQL query to the insertCustomer() method which executes it.
Reading the Data
This is the most important and widely performed operation. and you’ll see the records. } // Close the statements and Connections stmt. then assigns all the eight tokens in the record to appropriate variables. is very simple. if (con != null) { // Create statement from connection Statement stmt = con. if (result != -1) { System.println("Inserted" + result + "Record(s) successfully")..

Instead of specifying the column numbers. The get methods can either take the column number starting from 1 or the column name itself. we used the executeQuery() method to execute the SELECT query. In our example we used the column numbers. This is what the while loop does. } } }
As you can see from the above code. we need to read the record that the pointer points using the get methods.getString(3).getString(8). we can also specify the column names as shown below:
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. This method will return a ResultSet object as shown below:
ResultSet records = stmt.
A resultset object can be imagined like a table of records with a pointer at the beginning of the table as shown below: FirstName LastName SSN Age City State Country Joe Roberts 123456 20 Vegas CA USA Sam Perry 34534543 22 LA CA USA Serra Bates 567657 67 SFO CA USA To read the records.getInt(5). country = rs.getString(1).executeQuery(sql).getString(2). state = rs.println("Unable to get the connection").
Based on the above methods. lastName = rs. and all the remaining are VARCHAR types. city = rs.getInt(4). since SSN and Age are declared as NUMERIC data types when we first created the table. Once we are in the loop.getString(7).out. Following are the most commonly used get methods:
public String getString() public int getInt() public Date getDate() Used for VARCHAR columns Used for NUMERIC columns Used for DATE columns. ssn = rs.Chapter 11
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System. so we used getString() method to read the column data as shown below:
String String String String String String String String firstName = rs. we used getInt() method to read the data. we need to move the pointer to the record using the next() method until the pointer reaches the end of the records. age = rs.getString(6). middleName = rs.

The only difference is the execute method that we call. (Some records will be deleted).M.Denver. UPDATE etc) use the executeUpdate() method.USA Rob.3 with the above query.getString(“LastName”). Use the code in listing 11.3.Denver.). ‘Smith’.
You are free to use which ever convention you like.Perry.USA Sam.5 to see the updated data.Denver.43.
After running the the code in listing 11. If all goes well.Ball. you’ll see the following output:
John.Chapter 11
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String firstName = rs. The DELETE and UPDATE operations work the same way as CREATE and INSERT.33. run the code in listing 11. You can use the same program by just modifying the sql.20.Roberts.22.
UPDATE CUSTOMERS SET Country = 'Canada' where Age < 30. run the code in listing 11. For deleting data use the following query.getString(“FirstName”). INSERT.30.R. DELETE. Here is the trick to remember: Trick: If the query is SELECT.123123.USA
If you noticed carefully.
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. In our case.CO. Once we get the data.5 to see the updated data. INSERT INTO CUSTOMERS VALUE ( ‘John’.CO. The while loop will print all the records and terminates.
DELETE FROM CUSTOMERS WHERE Age < 30. we simply displayed it. most of the code is same in all the programs.ALewis.CO. For updating data use the following query. Take this as home work.P.890890. Compile and execute the code.567567.CO.USA Mike.Smith.USA Joe.7686456.3 with the above query.12345. we hard coded all the data in the query itself as shown below:
SELECT * FROM CUSTOMERS WHERE NAME = ‘sam’. If you observe the SQL queries we used in the above examples. we can do what ever we want.23432…. String lastName = rs.CO. For all other queries (CREATE.
After running the code in listing 11.12. but use the following queries.H.Denver. use the executeQuery() method.Denver.

Like the name suggests this is really a “prepared” statement. // Execute the query }
Here we wrote a method that takes the lastname and dynamically constructs the query by appending it at the end. Let see how we do this. The basic idea with this class is using the ‘?’ symbol in the query where ever the data is to be substituted and then when the data is available. The usage of this class is pretty simple. Populate the statement 3. These will not only simplify the process. As an example. The submitted data will then go to a JDBC program that reads the data and inserts it into the database. Create a PreparedStatement 2. let’s say we want to write a search application that searches for customers based on last name. the SQL query must be dynamically built as shown below:
void search( String lastName ){ String sql = "SELECT * FROM CUSTOMERS WHERE LASTNAME =‘" + lastName + " ' ". In this case. This class replaces the old Statement class that we used for executing the queries in the previous examples. Step1: To create a prepared statement we use the following method on the Connection object as shown below:
PreparedStament prepareStatement ( String sql ). quotes etc as shown above. One good example is registering in a web site where we fill in a HTML form and submit it. data should be inserted or retrieved on the fly based on the data supplied at run time.
PreparedStatement
In real world applications. it gets cumbersome while formatting the sql. This is where prepared statements come in handy. there is a better approach for executing the queries whose parameters are dynamic. Execute the prepared statement. Let’s see what it is. but also improve the performance of the application. replace the symbols with the data.Chapter 11
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In real world applications. We need to carefully use all the punctuation marks in the sql like the commas. Though the above approach works. It involves three simple steps as listed below: 1.
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. In JDBC. the data to the query comes from some other world.

we used eight question marks. stmt. there are no single quotes like we had in earlier examples.
Step 2: Now that we created a prepared statement. public class PreparedStatementDemo { public static void main(String args[]) throws Exception { // Pass the data to the method.7. Look at the following table that tells which method to use: Table 11. we create the prepared statement as shown below:
PreparedStatement stmt = conn. The first parameter to the set method denotes the position of ‘?’ to replace and the second argument is the actual data. name data). import java. ?. we use setString() method. prepareStatement ( sql ). Once we have the above sql.
package jdbc. its time to populate it with the data.sql. See the code in listing 11. ssn data).*.
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. If Database Column is VARCHAR NUMERIC or INTEGER DATE OR TIMESTAMP Method to use is setString() setInt() setDate()
In the above sql. Let’s write a simple example to get a clear picture. ?. ?. The trick here is.Chapter 11
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The SQL query to be passed to the above method will look as shown below:
String sql = “INSERT INTO CUSTOMERS VALUE ( ?.7 (PreparedStatementDemo. ?. which set method to use. To do this.
Since we need to insert data into eight columns. right? These work opposite to the get methods we used with ResultSet class couple pages back. ?. the first ‘?’ represents ‘firstName’ which is a VARCHAR. If you notice the query.java) JDBC program using prepared statement. we use the set methods as shown below:
stmt. ?)”. So. ?.3 Prepared Statement methods.
Looks like the above set methods are familiar.setString (1.setInt (3. Listing 11.

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. you’ll see the following output:
Inserted 1 record(s) successfully. stmt. String sql = “UPDATE CUSTOMERS SET FIRSTNAME= ? WHERE SSN = ?”. There is absolutely no query formatting at all. if we could send all the queries across the network in one go. and making the database run it. Look how clean this program is. you have to call the executeUpdate() method 1000 times. It would rather be good.setInt ( 2. if you have 1000 queries. you can call yourself a champion JDBC developer. If you understood the above examples.setString ( 1. To do batch processing using JDBC. we need to follow 2 simple steps: 1. read one by one. If you have several of these queries. and execute it.Chapter 11
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mark symbol but not the column number in the table. “Smith”). The main() method simply passes the data to the insertCustomer() method.
Following are some of the examples that demonstrate the usage of PreparedStatement class. Executing a single SQL query means. we are physically sending each query to the database possibly across the network (in real world applications. trust me. This process takes some time and resources. You can curse me if you see anything besides the above. The above examples are what you’ll see in most of the real world applications. what you can do is store the SQL queries in a file. The only difference is that you’ll see several of thse sql queries and several classes. // Used setInt since ssn is a number in the table
This completes all the important things that you need to know in JDBC.
Batch Processing
The JDBC programs we wrote until now simply execute one SQL query at a time. we need to call the executeUpdate() method for each and every query. stmt. This is what is called as batch processing of queries. stmt. “John”). If all goes well. Execute the batch. This method will then use its parameters to populate the prepared statement. and then have the database execute all the queries in one go. Add all the SQL queries to the batch 2. Compile and execute the code.
String sql = “SELECT * FROM CUSTOMER WHERE FIRSTNAME = ? “. right? So. setString ( 1. The only drawback with this approach is that. 87690 ). databases are distributed across the network).

Once all the queries are added.println("Something went wrong"). } } } else { System.addBatch(sql2).println("Processed SQL"). Compile and execute the code.addBatch(sql1). stmt. i < status. stmt.println("Unable to get the connection"). In such
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. for (int i = 0. i++) { if (status[i] != -1) { System. it defined six queries and then added them using the addBatch() methods.out. // Run the batch int[] status = stmt.addBatch(sql4).out. stmt. stmt. } } }
If you look at the runAsBatch() method.addBatch(sql5). // Add the SQL's to the Batch stmt.
Database Transactions
In some situations. processing a single business transaction might involve executing more than one SQL queries in which all the queries must either succeed or fail. If all goes well. let’s move on to some thing that is really important while working with databases.out. you’ll see the following output:
Processed Processed Processed Processed Processed Processed SQL SQL SQL SQL SQL SQL
With the above knowledge of batch processing.addBatch(sql6).executeBatch().Chapter 11
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con = DBUtils.addBatch(sql3). } else { System. if (con != null) { // Create statement from connection stmt = con.length.createStatement(). stmt. we used the executeBatch() method to execute all the queries in one go.getConnection().

Listing 11. Increment the funds in B’s account For the business transaction to succeed. If SQL1 succeeds and SQL2 fails for some reason. if both the SQL's succeeded. meaning. import java.
public void setAutoCommit() public void commit() and public void rollBack(). This financial transaction requires the following two basic operations. then we need to commit the transaction in which case changes should be made permanently in the database. See the code in listing 11. from business standpoint this means we debited funds from A's account but failed to credit B's account. To prevent this. group SQL1 and SQL2 as one operation (we call this as Transaction) and if any of the SQL fails then we need to rollback the transaction. public class TransactionDemo { public static void main(String args[]){ try {
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. In the JDBC Connection interface. SQL1 and SQL2 perform the above two operations.Chapter 11
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scenarios what we need to do is.java) JDBC program processing transactions. Let‘s say. group all the queries as one single unit of work and execute this single unit as a whole. say. However. there are three methods for transaction support namely.sql. the database went down. Such a unit is called as a transaction. Decrement the funds from A's account 2. Two customers go to bank to transfer funds from one account to another. Even if one fails. This is totally unacceptable and the customers will freak out. both the above two operations must succeed. Let's look at a real world scenario. bring back A's and B's accounts to original state that existed before the transaction began. what we need to do is.
Using the above methods. 1.*.
package jdbc.9 (TransactionDemo.9. let’s write a program and then see how it works. the entire financial transaction must fail.

conn.out. there will be multiple queries to execute as part of single transaction and are always run within a database transaction. Opening a connection at the beginning and closing the connection at the end when everything is done. Let’s say a database supports only 10 connections. }
In most of the real world applications. every database has limited number of connections. and it will not reuse this connection until the connection is closed. we are always closing the connection in the finally block. We always want to close the connection irrespective of whether the program runs or not.
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.executeUpdate(sql2). You need to shutdown the database and restart it to release all the connections which is not good. the 11th time you run.executeUpdate(sql1).Chapter 11
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The above statement will tell the database not to permanently make the table changes until the user informs it to do so. there is one thing that we are repeatedly doing all the times. there are some theoretical concepts that you need to now to be more comfortable. database will refuse to give you a connection.
We then check if the values of status1 and status2 are positive or not and based on it we either commit or rollback the transaction as shown below:
if (status1 != -1 && status2 != -1) { System. conn.println("Something went wrong! Rolled back the transaction"). They are simple though :-)
Connection Pooling
In all the programs we wrote thus far.commit().println("Joint account created"). and if your program doesn't close the connection every time it ran. There are two major problems that hunt us when we write database programs. Can you guess why we are doing this? You guessed it right.rollback(). } else { System. Read them like a story. This completes all the practicals that we need to do in this chapter. ok. If you noticed carefully. from database point of view somebody is still using the connection. We then execute the two queries using the executeUpdate() method as shown below:
int status1 = stmt. Problem 1: Usually. If you don't close a connection after using it. int status2 = stmt. However.out. You can stop this chapter here and can go into the next chapter.

Such a mechanism is what we call as connection pooling. It initializes a pool of connections with the database. Take a look at Fig 11. what will happen to the remaining 800 customers? They are simply kicked off. Who else other than the database vendors know better about connection details? This is the reason why database vendors themselves implement connection pooling mechanism in Java using certain standards. This is a big relief for us.
Connection Pool Program
Java Application
DB
Connection Pool Fig 11. The way the connection pooling works is. 1. it returns a connection from the pool. So. Problem 2: Let's say we have a database that can support a maximum of 200 connections at a time and the application we wrote supports 1000 customers at a time. Solution: Need to have a sophisticated mechanism to manage the connections effectively and efficiently by optimizing the usage of database connections. The application after using the connection returns it back to the pool. if the 200 connections are used for 200 customers. Sweet! Connection pooling mechanism addresses both the above problems. 2. right? The connection pooling program comes along when we download the drivers for a particular database.Chapter 11
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Solution: Always make sure you close the database connection. Now the question is who implements connection pooling? To implement the connection pooling we need to know all the intricacies of database connection.6 Connection Pooling
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.6 to get a clear idea. 3. This is no good because you are loosing business. When a connection is requested by the application.

Once we get a connection object. Since we are not yet there. before using a particular database say MySQL. resulting in the second transaction reading an invalid value. and initiates a transaction. Connection pooling is usually used with large scale enterprise applications where thousands of processes need to access the database at the same time. Dirty Reads 2. we need to check whether it addressed the problems or not. This is known as dirty read because there is always a possibility that the first transaction may rollback the change. Hmmm. Now the account will have $300 in it.
Database Isolation levels
I will conclude JDBC with this one last topic.
Dirty Read
Quite often in database processing. the rest of the program will be the same. For now understanding the details of connection pooling is good enough. In such situations the application server will be configured to use the connection pooling. At this point of time. and find out how much damage they may cause to our application. Since we are working with databases. we need to know some of the general problems that any database can have. we come across a situation wherein one transaction can change a value.Chapter 11
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Connection pooling will be implemented in such a way that all the details are hidden behind the scenes. Ex: Let's say we have a joint account with $500 in it. we will revisit this topic in the later chapters. Repeatable Reads 3. So. Phantom Reads Let's see what these are. The 3 most common problems with any database are: 1. Jim goes to the bank to debit $200. His transaction sees only $300 in the account and
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. and check whether the database vendors addressed the problems in their database or not. These problems may play a significant role in paralyzing your enterprise application. The only change we as application developers need to make is the way we retrieve the connection object. and a second transaction can read this value before the original change has been committed or rolled back. Tom goes to ATM to withdraw $400.

All this is to say that dirty reads don't cause damages to data.
Phantom Reads
'A' does a SELECT query on a table and gets 10 matching records. he will get different data. Tom checks the account balance and to his surprise sees $500 in it and freaks out. Few minutes later. 'B' modifies the table such a way that when ‘A’ queries the table again it would get different number of records back. what is happening is that the transactions are getting overlapped with each other.4 Isolation Levels Isolation Level 0 1 2 3 DirtyRead Addressed No Yes Yes Yes Repeatable Read Addressed No No Yes Yes Phantom Read Addressed No No No Yes
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. Every database will have something called isolation level that indicates which of the above problems are addressed. but they confuse the users. Now. TRANSACTION_SERIALIZABLE = 3. TRANSACTION_REPEATABLE_READ = 2. If 'B' reads the same row again.
Following table lists various isolation levels and what problems are addressed by various isolation levels: Table 11. So the solution is.
Repeatable Read
'B' performs a read. so his 200$ is credited back which leaves the account back at $500. check this isolation level and then decide whether or not we want to continue with our database transactions. The isolation level is a number from 0-3 (inclusive).Chapter 11
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gets rejected saying "Insufficient funds" which is OK. These are represented as CONSTANTS in the Connection interface as follows. to isolate the individual transactions. TRANSACTION_READ_COMMITTED = 1. but 'A' modifies or deletes that data later. What's going on? The good thing happened is the funds didn’t get lost.
public public public public static static static static final final final final int int int int TRANSACTION_READ_UNCOMMITTED = 0. In all the above three problems. All we need to do is. Jim changes his mind and cancels his transaction at the bank.

getTransactionIsolation() == Connection.TRANSACTION_READ_COMMITTED) { System. the best isolation level is 3 which address all the above problems. try { if (con.println("MYSQL database addresses DIRTY READS ONLY").out.TRANSACTION_READ_UNCOMMITTED) { } System.*. Following is the method in the Connection interface that returns the isolation level of the database."). } else if (con.println("MYSQL database addresses DIRTY READS.println("MYSQL database does not address anything. we want to set our own isolation level.Good"). } else if (con.out.getTransactionIsolation() == Connection.java) JDBC program checking the isolation levels.
Without wasting any further time. } else if (con.TRANSACTION_SERIALIZABLE) { System.
// // // //
con. we can use the following to only address "Dirty reads".TRANSACTION_REPEATABLE_READ) { System.getTransactionIsolation() == Connection.out.getTransactionIsolation() == Connection.setTransactionIsolation(Connection.Chapter 11
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Database Isolation levels
As you can see from the above table. import java. let’s write a small program to display the isolation of MySQL database.10. So the isolation level will always be 3. public class IsolationTest { public static void main(String args[]) { } checkIsolation().10 (IsolationTest. REPEATABLE READS and PHANTOM READS.sql.
public int getIsolationLevel(). Listing 11. See the code in listing 11.out.
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. Most of the databases will address all the three problems.
package jdbc.println("MYSQL database addresses DIRTY READS and REPEATABLE READS ONLY"). But many a times.TRANSACTION_READ_COMMITTED).
private static void checkIsolation() { Connection con = null.

Type-1.printStackTrace(). This completes all the topics in this chapter. If the concept like isolation levels is confusing. I am sure you are now an expert in writing JDBC programs to work with the databases. it always returns 3.
Summary
JDBC technology is used by Java applications to connect with database.9% of the enterprise applications are fine with isolation level 2 even if the database supports isolation level 3. It is seldom seen in real world applications. don’t worry about it.Chapter 11
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Summary
// Here you do your SQL operations. Compile and execute the above program to see which isolation level is supported by your database. Most of the databases address all the three problems. we need to select our own isolation level to improve the application performance. use the following method in the Connection interface. JDBC is a universal API for accessing any database. } finally { DBUtils. like creating statements and all // that good stuff. Isolation level 3 is termed as "very strong isolation" and poses serious performance problems. Type-3 and Type-4. when you try to get the isolation level using the above code. The important things that you need to understand is how to execute the SQL queries using Statement and Prepared Statement classes.
In our example we used the above method and changed the isolation level to 1.
public void setTransactionIsolation(int isolationLevel). So. 99. To change the isolation level. we got the isolation level using the getIsolationLevel() method and then checked which level of isolation is supported by the database using the conditional statements.
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. } } }
In the above code.closeConnection(). Type-2. There are four different types of database drivers namely.OK } catch (Exception e) { e. In such cases. Every database will have something called as Driver using which external applications can connect to database.

while the later compiles the query just once and execute ‘n’ number of times. PreparedStatement provides more flexibility to bind the data. The main difference between Statement and PreparedStatement classes is that Statement object always compiles the SQL before executing it. Because of this. Every database have problems like Dirty Reads. 3.
Time to play 50-50
1. b) DriverManager. Which of the following is a valid database URL?
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. Which of the following is used to load the database driver? a) Class.Driver”) b) Class. Phantom Reads. We use executeQuery() method to execute SELECT statements. Repeatable Reads. PreparedStatement offers better performance. JDBC applications need to load the driver before getting a connection to the database. Application Servers usually configure the connection pooling program. we can execute different SQL queries.jdbc.loadConnection().getConnection(). 2.loadClass(“com. we use connection pooling. We use CallableStatement to execute stored procedures. Using JDBC API such as Statement and PreparedStatement classes.Chapter 11
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Time to play 50-50
JDBC uses Pure Java Driver (Type-4) driver to connect with databases.Driver”). and upon using it returns back to the pool instead of closing the connection. Which of the following is used to retrieve a connection to database? a) DriverManager. The transaction will either commit or rollback based on the query results. We use the Isolation Level of the database to check whether the above problems are addressed by the database vendors or not.jdbc. Database transactions are used to group multiple queries into one single operation.mysql. To optimize the usage of connections in enterprise applications. An application retrieves the connection from the pool.mysql. Connection pooling maintains a pool of connections with the database. and executeUpdate() method for all other SQL queries.forName(“com.

Which of the following method returns a Statement object? a) createStatement() b) getStatement() 7. Which of the following method is used to execute an UPDATE query? a) executeQuery() b) executeUpdate() 9.Chapter 11
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Time to play 50-50
a) http:mysql://localhost:3306/MyDB b) jdbc:mysql://localhost:3306/MyDB 4. Which of the following method returns a PreparedStatement object? a) createPreparedStatement() b) prepareStatement(). Which of the following driver JDBC normally uses? a) Type – 1 Driver b) Type – 4 Driver
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. Which of the following interface represents a JDBC connection? a) JDBCConnection b) Connection 5. 8. Which of the following method is used to execute SELECT statement? a) executeQuery() b) executeUpdate() 10. Which of the following class is used for executing SQL queries? a) Statement b) SQLStatement 6.

Let’s move forward. PreparedStatement offers better performace. Pure Java Driveer. We are now ready to learn another simple technology used to build web applications. PreparedStatement is used for executing dynamic SQL queries where the data is plugged in at run time. while the later compiles the SQL just once and execute ‘n’ number of times. Question: Explain the various steps that to execute SQL queries using JDBC. Question: What is the difference between Statement and PreparedStament? Answer: Statement object always compiles the SQL before executing it. Answer: Refer to page 284. refer to page 280. The fun begins. Partly Native Partly Java driver. Based on the results. Question: What is a database transaction? Answer: A database transaction represents a group of SQL’s which either succeed or fail as a whole. This completed the JDBC chapter. I am sure you enjoyed this chapter more than anything else. the transaction is either committed or rolled back.
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. Because of this. Intermediate Server Driver. For details.Interview Questions JDBC
Interview Questions
Question: What are the different types of JDBC drivers? Answer: JDBC-ODBC bridge driver.

bat
and copy the following contents into it
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. 2.jar file into the already created j2eelib directory. Create a batch file named “env. This chapter will also give you an idea on parsing XML documents using Java. Copy C:\sun\AppServer\lib\xercesImpl.
Chapter Goals
Understand what XML is Understand the XML standards SAX and DOM XML parsing using SAX and DOM Understanding JAXP API for parsing XML documents
Environment Setup
1. Understanding this chapter is very important from J2EE point of view as XML is widely used in almost all the J2EE applications. 3.bat” as shown below
C:/JavaTraining/chapter12>env. you should be able to know what XML is and its usage in Java. Create directory ‘xml’ as shown below:
C:/JavaTraining/chapter12/xml
Save all the programs in this chapter in “xml” directory.Chapter 12
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Chapter Goals
Chapter 12
XML And Java
By the end of this chapter.

jar. C:\JavaTraining\j2eelib\j2ee.
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.c:\JavaTraining\ j2eelib\xercesImpl.bat
Once you execute the above command you are ready to compile and execute any programs in this chapter.C:\JavaTraining\j2eelib \mysql-connector-java-5.jar. Note: Whenever you start working with XML.jar..0.Chapter 12
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Chapter Goals
set CLASSPATH=. the first thing you need to do is execute the batch file as shown below to setup the CLASSPATH:
c:/JavaTraining>chapter12>env.3-bin.

Following is how a XML tag looks like:
<lastName>John</lastName>
In the above tag. Ever since its inception.
What is an XML Document?
We all know that there are several ways for storing data and information. Every tag has a beginning and an end. XML document is just another way for storing data and information but uses a tree like structure. An XML document/file comprises of several tags that represent the data or information. we use a text file to store the data line by line. lastName is the name of the tag.. All the characters or text in between represent the tag data The above tag is the simplest and smallest tag you can see in an XML document.
What is XML?
XML stands for “eXtensible Markup Language”. For instance. let’s see what this is and how we can use XML in the world of J2EE. However. The notion of XML is based on something called XML Document. we use database where tables are used to store data etc. we can also have complex tags which are nothing but tags within tags as shown below:
<name> <firstName>John</firstName> <lastName>Smith</lastName> </name>
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. Its adoption by the software industry is such that you’ll hardly find any software application that doesn’t use XML. The beginning of the tag is denoted by the tag name in between ‘<’ and ‘>’ and the end of the tag is denoted with the tag name in between ‘</’ and ‘>’ symbols. So.Chapter 12
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Introduction
Introduction
XML technology has become a buzz word every where in the IT community. These tags are also referred to as nodes. XML technology has made leaps and bounds and completely changed the way enterprise computing is done.

Following figure shows the tree representation of the above xml document:
Customer
FirstName
LastName
Age
Address
SSN
AddressLine1
City
State
Country
Fig 12.lastName. In this document.1 XML Tree
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. The good thing with XML is that. However.0"?> <customer> <firstName>John</firstName> <lastName>Smith</lastName> <age>20</age> <ssn>23324</ssn> <address> <addressline1>Apt 2222</addressline1> <city>Columbus</city> <state>OH</state> <country>USA</country> </address> </customer>
The above XML represents customer information. The address child node in turn has 4 child nodes. Following shows a well formed XML document:
<?xml version="1.ssn and address. This has five child nodes namely firstName. customer is the root node. Don’t you see a tree like representation here? I am sure you did. the tag names can be anything. Rule: Every tag that is opened must be closed. then we say that the XML document as well formed.Chapter 12
XML And Java
What is an XML Document?
Here. every tag has a closing tag which is why we say that it is well formed. If the above rule is followed.age. there is just one rule that we need to strictly follow. If you notice. name is a complex tag that contains two simple tags namely firstName and lastName. There is no limit on the number of child nodes a node can have.

A tag can have any number of attributes as shown below:
<book author="john" isbn="HG76876" pages="200"> <publisher>Wrox</publisher> </book>
An XML document is saved with the extension “. In XML.smith@abc.com”>
In the above node. If the document is well-formed.xml”. a node can also have attributes as shown below:
<customer email = “john. you’ll see the complete XML in the browser as shown below:
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. It represents the version of the XML we are using. email is the attribute of tag whose value is john.Chapter 12
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What is an XML Document?
I know you guys would be thinking what the following line is
<?xml version="1.smith@abc.com.0"?>
We call this as the prolog of the XML document. One simple way of verifying whether an XML document is well-formed or not is by opening the xml file using any browser like Internet Explorer.

This is the reason why XML though very simple. is yet so powerful. The only rule is that the document must be well formed. Since enterprise applications involve complex data representation and processing. you’ll see how J2EE extensively uses XML in various ways for simplifying things.
Why XML is important?
The main reason why XML has tasted unprecedented success is because it is 100% language independent and platform independent. It can be used to represent any complex data or information without any limitations on size with user defined tags. The error will also list the element name where the violation occurred.Chapter 12
XML And Java
If we fail to close some tag. the flexibility that XML offers make it an ideal candidate for using in such situations.
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. Over the course of next several chapters. the browser will display an error as shown below indicating that the XML document is not well formed.

the first thing we need to do before processing the data is to verify whether the data is valid or not. A DTD defines the validation rules for elements in an XML document using the ELEMENT declaration. The syntax for the element declarations in a DTD is shown below:
<!ELEMENT element name content-model>
The content-model basically tells the type of content the element can have. EMPTY: This indicates the element will not have any content As an example. and the element usage in the XML based on the DTD declaration DTD
<!ELEMENT format EMPTY>
XML
<format></format> or <format/>
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. There are four types of content models an element can have as listed below: 1. look at the following DTD for an element.
Document Type Definition (DTD)
A DTD basically defines the following things: 1. Let’s see how we define an xml element using a DTD declaration.Chapter 12
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XML Validation
XML Validation
As I said before XML is used to represent data or information.dtd” and is referenced by the XML document. Whenever we have any data. The valid list child elements of a particular element 3. So. let see how to use DTD to validate data. The list of valid attributes for a particular element All the validations defined using DTD are stored in a file with the extension “. The order of elements in the XML 2. right? So the question is how to validate the data represented by XML document? The simplest way is by using a DTD which stands for Document Type Definition.

DTD
<!ELEMENT description (#PCDATA|b|code)* >
where asterisk(*) indicates that the elements in parenthesis can occur 0 or more times within description. See the following DTD and equivalent XML. country) > addressLine1#PCDATA >
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. city.23</accountBalance> </account>
4.address) > firstName #PCDATA > lastName #PCDATA > age #PCDATA > ssn #PCDATA > address (addressLine1. ANY: This indicates that the element can have anything.lastName. accountBalance) >
XML
<account> <accountNumber>1234</accountNumber> <accountBalance>100. Following lists the various symbols. DTD
<!ELEMENT account (accountNumber. Children Only: This indicates that the element can only have child elements in the specified order.Chapter 12
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XML Validation
2. Look at the following DTD and equivalent XML. + means 1 or more ? means 0 or 1 * means 0 or more XML
<description> This is a test <b> description </b> The child elements can be <code> in </code> in any <b>order</b> </description>
Using the above declarations. state.
<!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT <!ELEMENT customer (firstName. following is the DTD for the customer data we defined before. Text with mixed children: This indicates that an element can have text as well as specified children in any order. 3.age.ssn.

The address element in turn defines its child elements along with the order of the elements. So. </customer>
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. It then defines the content types of every element. publisher) > <! ATTLIST book isbn CDATA #REQUIRED pages CDATA #IMPLIED >
The above declaration tells that book element can have two attributes namely isbn which is required and pages attribute which is optional.age. All the text elements are defined as PCDATA which stands for Parsed Character DATA. the xml will look as shown below: XML
<book isbn="SD34324" pages="100"> <author>James</author> <publisher>BPB</publisher> </book>
These are the basic things you need to know to write a DTD for an XML document.dtd”> <?xml version=”1.lastName.address in the same order.
<!ELEMENT book (author. As we learned before.ssn. Let’s see how we can define the validation rules for attributes using DTD. an XML element can also have attributes. Once we have the DTD in a file say customer.Chapter 12
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XML Validation
<!ELEMENT city #PCDATA > <!ELEMENT state #PCDATA > <!ELEMENT country #PCDATA >
The above DTD tells that the customer element should have the five child nodes namely firstName.dtd. we finally need to link it with the XML file using the DOCTYPE element as shown below:
<!DOCTYPE customer SYSTEM “customer. We’ll see what parsing is in the next section. Following is the syntax for attribute declarations in a DTD
<! ATTLIST element-name (attribute-name attribute-type default-declaration) * >
Consider the following DTD definition for an element named book.0” ?> <customer> ……….

1 XML Schema Data types Data Type string boolean float double Description Used for text data Used for boolean data (True/False) Used for 32 bit decimal numbers Used for 64 bit decimal numbers
Using the above data types. an element named score will be defined as shown below:
<xsd:element name=”score” type=”xsd:int”/>
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. Table 12. First. Once the XML is linked with the DTD. 3.Chapter 12
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XML Validation
It’s important that you have the DOCTYPE element before the XML prolog. let’s look at the important simple data types listed in the following table. One of the major concerns with DTD is the lack of support of various data types. 2. a new validation scheme is created which is called as XML Schema. No support for validating numeric and boolean data To overcome the above limitations. There is no way that using a DTD we can validate the type of data an element can have. It contains several built in primitive data types such as string. 1. Though DTD provides a simple way of validating an XML document. This is where schema comes in real handy. XML Schemas overcame all the limitations of DTD and had now become the standard for validating XML documents. The good thing about XML Schema is that it is closely associated with Object Oriented data models. It can only validate the order of the elements. it has certain limitations listed below. attributes etc. an XML Schema is itself an XML document with elements.
XML Schema
Unlike a DTD. our applications can start validating the XML document before processing it. It cannot validate the list of valid values an element can have. Less flexible. float etc for validating the data. A DTD cannot be extended using inheritance. Let’s see what this is and how we can use xml schema to validate xml documents in a better way. XML Schema can also be used to build complex data types using simple data types. integer.

The XML defines several complex data elements namely garage. Let’s now define definition for the root element rentalcompany.apartment etc.garages. leasecontract: Following is the schema definition for this element. it only has attributes which are defined by the <xsd:attribute> elements. garage : Following is the schema definition for this element.leasecontract.apartment etc. The minOccurs attribute makes the element optional. Let’s look at one by one. complete the definitions for other elements namely address. rentalcompany: Following is the schema definition for this element. Following similar lines. The sequence element tells the schema about the order of the elements in the XML document.
<xsd:element name=”garage”> <xsd:complexType> <xsd:attribute name=”id” type=”xsd:string”/> <xsd:attribute name=”rate” type=”xsd:float”/> </xsd:complexType> </xsd:element>
The <xsd:element> tag defines the name of the element as garage. The <xsd:complexType> element tells the schema that the element named garage is a complex element that can have both child elements as well as attributes.
<xsd:element name=”rentalcompany”> <xsd:complexType> <xsd:sequence> <xsd:element ref=”type”/>
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. It then built the complex data type named leasecontract by referencing the simple elements. In our case.
<xsd:element name=”leasingperiod” type=”xsd:int”/> <xsd:element name=”lesse-name” type=”xsd:string”/> <xsd:element name=”deposit” type=”xsd:float”/> <xsd:element name=”leasecontract”> <xsd:complexType> <xsd:sequence> <xsd:element ref=”leasingperiod”/> <xsd:element ref=”lesse-name”/> <xsd:element ref=”deposit” minOccurs=”0”/> </xsd:sequence> </xsd:complexType> </xsd:element>
This element first defined the simple elements namely leasingperiod. deposit etc. The absence of this attribute means the element is required.Chapter 12
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XML Validation
Let’s define the schema for the above XML.

The only thing you need to
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. You don’t have to remember the syntax and all that. A namespace is defined using the attribute xmlns:xsd. Usually.w3. You can take my word. we are now ready to look at the most important aspect of XML. there will be several tools that automatically do it for you.0” ?> <xsd:schema xmlns:xsd=http://www. This tells the schema to look for the vocabulary for all the elements prefixed with xsd at the resource specified by the URI. you’ll still be fine. You are free to use which ever prefix you want. XML Parsing.org/2000/10/XMLSchema> <xsd:element name=”rentalcompany”> <xsd:complexType> <xsd:sequence> <xsd:element ref=”type”/> <xsd:element ref=”owner”/> <xsd:element ref=”address”/> <xsd:element ref=”apartments”/> <xsd:element ref=”garages” minOccurs=”0”/> </xsd:sequence> </xsd:complexType> </xsd:element> ……………. Even if you don’t understand the above XML schema and DTD. we need to assemble them into one big chunk of document called as schema document as shown below:
<?xml version=”1. I think with all this basic idea about DTD and XML Schema. we defined all the child elements under rentalcompany element.Chapter 12
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XML Validation
<xsd:element <xsd:element <xsd:element <xsd:element </xsd:sequence> </xsd:complexType> </xsd:element>
ref=”owner”/> ref=”address”/> ref=”apartments”/> ref=”garages” minOccurs=”0”/>
As you can see from the above chunk. </xsd:schema>
The important thing to note in the above schema document is the <xsd:schema> element which defined the namespace for all the elements in the schema. A namespace uses a prefixing strategy that associates element names with a Uniform Resource Identifier where the actual vocabulary for the elements will be defined. Once all the elements are defined. Namespaces come very handy especially when combining more than one XML document together resulting in namespace collisions. …………… ……………….

The application that parses an XML document is called an XML parser. 3. The purpose of an XML parser is to make some interfaces available to an application so that it can modify and read the contents of an XML document. Because of this reason. Moreover. SAX parsing is based on event model in which sequences of events are generated for each and every tag in the XML document. Though XML provides infinite flexibility to represent data of any complexity. This is where parsing an XML document comes into picture. Based on the type of events. 4. In DOM parsing. The various events a SAX based parser generates are: 1. SAX parsing is more efficient in situations where the XML document is huge and we only want to extract a small piece of data from it. In this model.
SAX
SAX is abbreviated for Simple API for XML. which I am sure you have absolutely no problem in writing it. This is the other side of XML.Chapter 12
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XML Parsing
know is writing a well formed XML document.
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. the application can take appropriate action. The generation of these interfaces is based on two XML standards namely SAX and DOM.
SAX based parsing can be used only for reading the data from XML and not for modifying the contents. an XML document is represented as a tree of nodes. it’s of no use if we cannot read the data back from XML file.
XML Parsing
Representing data using XML is one side of the coin. Start of Document Start of Tag End Of Tag End of Document etc. Parsing an XML document is nothing but reading the data back from the document. Good. the entire XML must be loaded into the memory before reading or modifying the document. A parser based on this model.
DOM
DOM stands for Document Object Model. DOM based parsing should be used only when the XML document is small enough not to cause any memory issues. 2. can traverse the through the nodes to read the data or modify the data by removing the nodes.

Once the xerces parser is loaded in the classpath. there is one limitation here. This is shown the environment setup at the beginning of the chapter. and the two parsing standards SAX and DOM. The five most important events are listed in the following table. Table 12. Now that we know what XML parsing is. If this is the case. One important thing to understand with JAXP is that it provides classes and interfaces that have zero implementation.jar.
SAX Parsing using JAXP
As said before. all we need to do is put it in the classpath. let’s see how we can use an XML parser it to parse XML documents from Java applications. we can use the JAXP API to write Java programs and parse XML documents using SAX and DOM. To use this jar file. Sun distributes this parser in the form of a Jar file named xercesimpl. JAXP uses the classes that are implemented by some other vendors to parse the documents. This API is created by Sun Microsystems as a universal way for parsing XML documents from Java. This is like if B adheres to A’s rules. This is where we need to know something about JAXP. we will use sun’s xerces parser which adheres to JAXP specification. we will see examples using both SAX as well as DOM. In this chapter. how can we use JAXP to parse the XML documents? Good question. SAX based parsing is based on event model in which a sequence of events are generated for processing the tags. Here B is the vendor XML parser and A is JAXP. There are several XML parsers available in the market for free of cost.2 SAX Events Event Description startDocument This event indicates the beginning of the XML document
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.Chapter 12
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JAXP
Any XML parser that is constructed will be based on either SAX or DOM model. However. In this chapter. Most notable ones are the parsers from Apache.
JAXP
JAXP stands for Java API for XML Parsing. In this chapter. then A can use B. we use the parser from Apache called as Xerces parser to parse the XML documents. BEA and Sun Microsystems. The vendors who implement the parsers must adhere to JAXP specification. This parser supports both SAX and DOM parsing.

and begins generating the events and send them to a handler that handles the events. Write a handler class to handle events. Java application uses a SAXParserFactory to get a SAXParser 2. Following the above steps.
Java Application S A X P A R S E R
startDocument() startElement() characters() Handler endElement() endDocument()
SAXParserFactory
XML
From the above figure. Write a class to initiate parsing with the following steps a) Get an instance of SAXParserFactory class b) Get a SAXParser from the factory class c) Invoke the parse()method by passing the XML file and a reference to handler object. we observe the following things: 1. Steps for parsing a document with SAX Parser 1. let’s write an example to parse the following XML:
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. The SAXParser takes the XML document to parse. 2.Chapter 12
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JAXP
startElement characters endElement endDocument
This event indicates the beginning of a tag or a node This represents the data within the tag This indicates the end of the tag This event indicates the end of the XML document
Following figure gives a complete idea of SAX Parsing.

The startElement() and endElement() methods have a name parameter which represents the name of the tag that is currently parsed. So what we will do is.out.java C:/>JavaTraining>chapter12>java xml. The only methods we use in this example are the characters() and endElement(). the output of the above program will be
John 23324
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. This is one of the JAXP built-in library classes.getCustomer(). The bean class is a simple class that defines the getters and setters for storing the data that is retrieved from the xml document. This method takes two parameters. The handler class should overwite the methods namely startDocument(). // Print the data in the bean System. These methods are known as callback methods that get invoked automatically when the parsing begins. When the end of the document is reached. a handler class MySAXHandler that handles the events and finally the SAXParserDemo that initiates the parsing. 1. When all the parsing is completed it retrieves the customer bean and displays the data. followed by characters() and then followed by the endElement()method. it first calls the startElement() method. check whether the name of the tag is either SSN or firstName and populate the customer bean with the data read in the characters() method. the customer bean will have both the SSN and the name populated. and initiates the parsing by invoking the parse() method.SAXParserDemo
If all goes well. The class must extend the HanderBase class. The SAXParserDemo class simply follows the steps outlined before. One is the name of the XML file.getSSN()). System. Compile and execute the programs using the following commands:
C:/>JavaTraining>chapter12>javac xml\*. A Customer bean for storing the data (firstName and SSN). startElement() etc.println(customer.Chapter 12
XML And Java
JAXP
Customer customer = handler.out. To write a handler class. } }
The above code comprises of 3 classes.getName()). 2.println(customer. and the other is the object of the handler class. Whenever the parser comes across a tag. we need to do the following 2 things.

The main disadvantage with SAX is that. So. Here the customer tag is repeating twice. We call this entire tree as the Document. The most important interfaces we use in DOM parsing are the Document and Node.
DOM Parsing using JAXP
In DOM Parsing. These are: 1. CDATA Node etc. 3. 2. Invoke the parse() method on the builder object by passing the XML file. Get an instance of DocumentBuilderFactory class. Attribute Node 3. In DOM. look at the following XML:
<customer> <name>John</name> <ssn>12345</ssn> </customer> <customer> <name>Sam</name> <ssn>4543</ssn> </customer>
The above XML represents data for two customers. the entire XML is loaded in the memory as a tree of nodes. the application logic will get more complicated due to duplicate elements. then extracting the data becomes really complicated. This method then returns a Document object. and then add another if block in the endElement() method to populate the beans property. Create a DocumentBuilder object using the factory class.Chapter 12
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JAXP
This is how we do SAX parsing. If you need any other data from XML. If we need to extract this customer data using SAX parser. The basic process to implement DOM parsing is shown below: 1. For the first time it might seem confusing. For instance. there are different types of nodes corresponding to the component types of an XML document.
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. Element Node 2. Once the XML is loaded. DOM parsing offers more flexibility. but go through it 2-3 times and you’ll get it. you will add getters and setters in the Customer bean. if the XML has repeating elements. Most of the real world applications implement SAX parsing just like the above example. In such situations. let’s see what it is. You can use the above code as a reference to write any Java program that needs to parse XML using SAX. we can traverse through the entire tree and get the information about every node.

you can use them as references.
Difference between SAX and DOM
SAX SAX is based on event model. b) It will iterate over the node list using for loop. no one will ever ask you to write code with a closed book. This is all you need to know about XML and XML parsing. You are free to use references available online or some other place and parse the XML documents as you wish. SAX generates various events sequentially like startDocument. it gets a Document object. get the customers information using the getData() method. startElement. and check the contents of the customers table. You can use this as a reference to parse an XML document using DOM. Even if you didn’t thoroughly understood the above two examples. populate the customer bean and finally store the bean in an ArrayList.Chapter 12
XML And Java
JAXP
a) Using the DOM API. SAX will never load the XML into memory. Save both the programs and compile them as shown below:
c:/>JavaTraining>chapter12>javac xml\*. and see two records inserted into the Customers table in the database. Read the following differences between the two. From interview standpoint. When you work in a company. SAX uses a handler to handle the events. endElement. and inserts the customers beans into the database. which uses the JDBC API. characters. c) It then passes the list of customers to saveToDatbase() method. and you are good to go.
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.java c:/>JavaTraining>chapter12>java xml. open DBVisualizer tool. you’ll see the following output. It will then retrieve the two customer nodes. all you need to know is the difference between SAX parsing and DOM parsing. That’s it. This is one of the widely used in real world applications. Most of the real world applications use either SAX parsing or DOM parsing based on the application.DOMParserDemo
If all goes well.
XML Data saved in the Database
To verify. endDocument.

DOM DOM is tree based. SAX is used for reading small portion of information from large XML documents. It uses parser implementations provided by third party vendors to parse documents. DOM is usually used with small XML documents. SAX and DOM are two standards for parsing XML documents. SAX parsing is based on event driven model and DOM is tree based. Xerces parser from Apache is one of JAXP complaint XML parser that supports both SAX and DOM parsing. I am sure this chapter may have helped you to know some basics about XML and its intracacies. DOM will load the entire XML into memory before parsing. Any application can use XML to exchange data.Chapter 12
XML And Java
Summary
SAX parsing is used for reading XML documents only and cannot be used to modify its contents. An XML document is said to be well-formed when every opened tag has a closing tag. So. JAXP API is used by Java applications for parsing XML documents. Question: What are two XML parsing standards? Answer: SAX and DOM
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.
Interview Questions
Question: What is an XML parser? Answer: A parser is an application used for reading and modifying the contents of an XML file. let’s summarize what we learned in this chapter. DOM is used to read and modify XML data. XML parser is used to parse XML documents. An XML document is built using tags. XML is 100% language and platform independent. JAXP is a specification.
Summary
XML is an industry standard for representing complex data.

This completes all the basic XML that we need to know to get on with J2EE. DOM is normally used with smaller XML files. Question: Which API is used for parsing XML documents? Answer: Java API for XML Processing (JAXP) Question: Which is the popular JAXP complaint XML parser ? Answer: xerces parser from Apache Software foundation. if you want to know more about XML. please refer to the books and resources that teach XML and XML related technologies in detail.Chapter 12
XML And Java
Interview Questions
Question: What is the difference between SAX and DOM? Answer: Refer to the last section. Let’s continue with J2EE for now. Question: When do you use SAX and DOM? Answer: We use SAX when a small portion of information is to be retrieved from a large XML document. We use DOM when we want to modify or read the entire content of the XML file.
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. However.

starting with this chapter. 2.
Chapter Goals
Understand the evolution of Web Applications Understand the basics of HTTP Understand the notion of Web Containers Servlet API Structure of J2EE Web Applications Building Web Applications using Servlet API Understand Web deployment descriptor
Environment Setup
1.Chapter 13
Servlet Programming
Chapter Goals
Chapter 13
Servlet Programming
This chapter introduces you to the world of Web Applications using Java. Since Web Applications form the basis for enterprise internet applications. Install Tomcat Server following the instructions at the end of the book. This first chapter introduces you to basic web application development using Servlet technology and also gives you an idea about Web Containers. Create the directory structure shown in the rectangle in Tomcat installation directory
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. the next few chapters give you all the details about Web application development using J2EE and open-source technologies.

bat” file in the following directory.
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This completes the setup for running the web applications:
Starting the Server
To start the server. Add the following variable to the system environment variables.1.31\bin
A command window opens and keeps listening.Chapter 13
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Chapter Goals
5.1.
CATALINA_HOME = c:\jakarta-tomcat-4.
c:\jakarta-tomcat-4. click on the “startup.

A general purpose browser like Internet Explorer to send requests and display the response. FireFox etc.Chapter 13
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Introduction
Introduction
Several years ago. 1. web clients are referred to as thin clients. use the following: 1. HTML markup for defining the user interfaces for interaction with the server 3. the layout of the user interface can also be controlled from the server side. Web clients to interact with server side applications. let’s first know some basics about it and then look at the server side details. Since the presentation logic is also moved to the server side. both the application logic and presentation logic are physically separated from the clients PC and moved to the server side. The server side program takes the HTTP request. client-server applications have become very popular for building enterprise applications.
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. Since the application logic is no longer present on the client PC. The Web client delegates all the user interactions to the server side application logic which then processes the requests and uses the presentation tier components to send the response. formats the HTML and displays it to the user. Web client then reads the HTTP response. processes it and sends a HTTP response back to the web client. With the advent of internet. Web client like IE sends a request using HTTP protocol. HTTP protocol to send and receive requests and responses. The way the web applications work is. 3. which then sends requests to the server application via network. client applications usually contained most of the presentation and application logic and the interaction with the server application is done through a GUI. A web client in this case is nothing but a browser application like Internet Explorer. Since HTTP plays an important role in web applications. Applications that use web clients for user interaction are called as Web Applications. client application is usually installed on the client’s personal computer. Any change in the application logic requires reinstallation of the client application on all the computers. 2. In this model. The response includes body content in HTML format. 2. In this model. application clients are completely replaced with web clients. In such a model. Such application clients are referred to as fat clients since the application logic is also included in the client application.

Each name-value pair must be separated by ‘&’ symbol. some data must be sent in the request and GET request can send the data by appending it in the URI itself as shown below:
http://somedomain.com?uid=John&role=admin
In the above URI. It is a stateless protocol. This doesn’t mean that GET requests cannot be used for retrieving dynamic resources. Based on how the data is sent in the request. A typical HTTP request is identified by something called URI (Uniform Resource Identifier) as shown below:
http:// <host name>:<port number>/<request details>
GET Request
GET request is the simplest of all the requests. processes it and sends the response back. This type of request is normally used for accessing server-side static resources such as HTML files. meaning that every HTTP request is independent of each other.Chapter 13
Servlet Programming
HTTP
HTTP
HTTP stands for Hyper Text Transfer Protocol. When secure information need to be sent. the HTTP request sends two pieces of data in the form of name value pairs. GET request cannot be used. GET requests can only send text data and not the binary data. Therefore in situations where you need to upload image files to server. and send a dynamic response. This is the most important feature of HTTP.
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. images etc. To retrieve dynamic resources. All the data in the request is appended to the URI itself making the data visible to every one. The server side program will then read the data. GET request is not the solution. Though GET request is simplest of all. 2. HTTP requests are categorized into several types. process it. but the most important and widely used are the GET requests and POST requests. The server side program reads the data. Following are some of the important properties of HTTP: 1. it has some limitations: 1. the ability to send data to server side program. 2. It can send data in the request.

the container is ready to receive HTTP requests and delegate them to the programs that run inside the container. Let’s see what this is.
POST Request
POST request is normally used for accessing dynamic resources. Let’s see each of them one by one. These programs then process the requests and generate the responses. POST requests are used when we need to send large amounts of data to the server. Moreover. J2EE offers the following: Servlet and JSP technology to build server side programs and Web Container for hosting the server side programs. XML files. let’s move on to the server side of the web applications. Deployment descriptor which is an XML file used to configure the web application. Now that we know the basics of HTTP.
Web Container
A Web container is the heart of Java based web application which is a sophisticated program that hosts the server side programs like Servlets. all the HTTP requests are sent as POST requests. Once the Servlet programs are deployed in this container. In most of the real world applications. Look at the following figure that shows how a typical web container looks like:
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.Chapter 13
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Server Side of the Web Application
These limitations are addressed by the HTTP POST request. Also a single web container can host several web applications.
Server Side of the Web Application
The server. the data in the POST request is hidden behind the scenes therefore making data transmittal more secure. Images etc Server programs for processing the HTTP requests A runtime that executes the server side programs A deployment tool for deploying the programs in the server In order to meet the above requirements.side is the heart of any web application as it comprises of the following: Static resources like HTML files.

I know you might be wondering about the need for several technologies to build web applications. These are Servlet and JSP technologies. you’ll get the answer by yourself as we move on.1 A typical web container
There are several J2EE Web Containers available in the market.
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. One of the most notable one is the Apache’s Tomcat container which is available for free of cost.Chapter 13
Servlet Programming
Server Side of the Web Application
Web Container
Web Application 1 Servlets Web Application 2 Servlets
JSP’s
JSP’s
HTML/Images
HTML/Images
XML
XML
One Deployment file
One Deployment file
Fig 13. Installing Tomcat Web container is one of the steps in the environment setup page at the beginning of the chapter. In this chapter. We need to have a programming model to build server side programs that run within this container. Having just the web container is not sufficient. Don’t worry. we will use Servlet technology to build web applications. J2EE supports two key technologies to build server side components in Java. Sweet! This is what we will install for running web applications.

Structure of a Web Application
For the web container to run the web application. With the above directory structure.xml classes myservlets Servlet1. Following table lists the purpose of each directory: Table 13.html ….class . Let’s say we want to build a web application named myweb. please install Tomcat Web container.class Servlet2. all the components within myweb application should be accessed with the URL starting with:
http://localhost:8080/myweb/
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. .
where webapps is a standard directory within the web container (Tomcat installation directory). It is from this directory. The directory structure for this application must be as shown below:
webapps myweb WEB-INF web.xml This directory must contain all the Java class files in the web application
All the static resources namely html files. Creating this directory structure is one of the steps in the environment configuration section at the beginning of the chapter. you need to start creating the directory structure. lib index.1 Web application directories Directory myweb WEB-INF classes Description This directory represents the entire web application This directory contains the configuration file like web. image files can be stored directly under myweb directory.Chapter 13
Servlet Programming
Server Side of the Web Application
Note: At this point. the web application must follow a standard directory structure within the container. since we know what a web container is..

2 Servlet API Class/Interface HttpServlet Description The base class that represents a HTTP Servlet The class that encapsulates all the HTTP request HttpServletRequest details The class used for sending HTTP response back to HttpServletResponse the browser ServletConfig Class used for dynamically initializing the servlet HttpSession Class used for session management Class used by the servlet to dispatch the request to RequestDispatcher various resources.2.Chapter 13
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Servlet Technology
If you want to create another web application like mywebproject. you must again create the same directory structure with mywebproject as the root directory and should access the components using the URL starting with:
http://localhost:8080/mywebproject/
At this point please make sure you created the directory structure as outlined in environment configuration section. Table 13.
Definition of Servlet
A server side Java program that uses the above API is called as Servlet. we are interested in the most important ones shown in Table 13. These classes and interfaces form what we call as Servlet API. In simple terms.
Servlet API
Though there are several classes and interfaces in this API. a Servlet is a server side Java program that processes HTTP requests and generates HTTP response.
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.
Servlet Technology
Servlet technology is a standard J2EE technology used for building dynamic web applications in Java. Using Servlet technology is again nothing but using the standard classes and interfaces that it comes with.

but it’s a good practice to have the class name as the servlet name. However.FormProcessingServlet</servlet-class> </servlet>
The servlet name can be any arbitrary name. This is nothing but the deployment descriptor. This file defines the following important information pertaining to a Servlet: 1.xml that is used by the web container to run the web applications. The URL mapping used to access the Servlet Let’s assume we wrote a servlet named FormProcesssingServlet in a package named myservlets. Are the above two things good enough to build applications? The answer is big No. A Web Container like Tomcat to run the server-side programs. it is the deployment descriptor that tells the web container what servlets to run. 2.
Deployment Descriptor
A deployment descriptor is a standard XML file named web. we have the following two important things to build the web applications in Java: 1. Once we have a Web container and several Servlets. This completes Step 1. Servlet name and Servlet class 2. Let’s see what this is. So.xml file).xml will be as shown below:
<servlet> <servlet-name>FormProcessingServlet</servlet-name> <servlet-class>myservlets. The next thing we need to define is the URL mapping which identifies how the servlet is accessed from the browser. For the above servlet. the servlet class tag must represent the fully qualified name of the servlet which includes the package name as shown above.Chapter 13
Servlet Programming
Deployment Descriptor
At this point. this is the key component of any web application. We are missing the most important component here. the url mapping will be as shown below:
<servlet-mapping> <servlet-name>FormProcessingServlet</servlet-name>
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. Servlet Technology to build the server-side programs. Every web application will have one and only one deployment descriptor (web. The definition for this servlet in the web.

We do all the initialization here.This //method can only process POST } }
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.FormProcessing Servlet class to process the request. the FormProcessingServlet should be accessed with the following URL:
http://localhost:8080/myweb/FormProcessingServlet
The web container then delegates the request to myservlets. HTTPServletResponse res) throws ServletException. } public void doGet(HTTPServletRequest req.io. Following is how a typical servlet looks like.servlet. That’s it.IOException { // This method gets called automatically for POST requests. This //method can only process GET } public void doPost(HTTPServletRequest req. public class TestServlet extends HTTPServlet { public void init(ServletConfig config) throws ServletException { // This method is called before the following methods are called //and gets called only ONCE. import java.
Steps for Writing a Servlet
Writing a servlet is very simple.http. This is how we bind the url mapping with a servlet.IOException { // This method gets called automatically for GET requests.*. Create a class that extends HttpServlet 2. doGet() and doPost(). the servlet name in both the XML snippets is the same. The url pattern defines how the servlet will be accessed. With the above mapping.
import javax.servlet. HTTPServletResponse res) throws ServletException.*. You just have to follow a standard process as shown below: 1.*.Chapter 13
Servlet Programming
Steps for Writing a Servlet
<url-pattern>/FormProcessingServlet</url-pattern> </servlet-mapping>
If you noticed carefully. This is like a // constructor. Trust me. import javax. Define 3 methods namely init().

take a look at the code in listing 13. Listing 13. The class extends HttpServlet 3.1 (GreetingServlet. Following is what a web container does with a servlet: 1. For all the GET requests. remember the following three important points: 1. the web container invokes the doGet() method. it does the following things 1.doGet() and doPost() These three methods are the standard methods for any servlet.*.
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.servlet. import javax.
LifeCycle of a Servlet
The life cycle of a servlet represents how the web container uses the servlet to process the requests. These are called as callback methods that the web container invokes automatically when a HTTP request comes to this servlet. let’s begin writing servlet examples. 3. Loads the Servlet Instantiates the Servlet Initializes the servlet by executing the init() method. it’s always a GET request 3.servlet. 2. and for POST requests it invokes the doPost() method. 4.Before we understand the details about how it works. import javax. Imports all the servlet classes 2. When a hyperlink is clicked. 5. Invokes the doGet() or doPost() methods to process the requests.http. 6.Chapter 13
Servlet Programming
LifeCycle of a Servlet
If you look at the above servlet.1. The first example we are going to write simply displays a greeting in a web browser. Repeats Step 4 until all the requests are processed Destroy the servlet
Before we start writing examples. Defined 3 methods init(). Any request sent by typing the URL in the browser is always a GET request 2.*.java) A simple servlet that echos a greeting
package myservlets. When a html form is submitted it can either be GET or POST Assuming that you completed all the steps in the environment setup section.

public class GreetingServlet extends HttpServlet { public void doGet(HttpServletRequest req.getWriter(). This method takes two arguments namely HttpServletRequest and HttpServletResponse. } }
This is a simple servlet which defined just the doGet() method since we will only send a request from the browser which is a GET request. Open a stream/channel to the browser to send the response.
3. 2.println("<h1>Welcome to Servlets </h1>").setContentType("text/html").println("<h1>Welcome to Servlets </h1>"). Save the servlet program in the following directory as
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.io. // Send the message pw.setContentType("text/html"). In this example we only use the response object to send a greeting message to the browser. To send a response.*. we need to do three things as outlined below: 1. IOException { res. Define the type of the response data. The former is used to read the data from the request and the later is used to send the response back to the browser.
The above line specifies that we are sending HTML response back. This is done using the following statement:
PrintWriter pw = res. This is done using the following statement:
res. To send response message to the browser.Chapter 13
Servlet Programming
LifeCycle of a Servlet
import java. we use the println() method as shown below:
pw. Steps to run the Servlet 1. Let’s now run the servlet. Send the html message to the browser.getWriter(). PrintWriter pw = res.
The first two steps are common to any servlet. HttpServletResponse res) throws ServletException.

The other advantage with this is. Servlet initialization is done in the init() method of the servlet. you only have to change the web. The web container while loading the servlet. right? The next example demonstrates servlet initialization.Driver</param-value> </init-param>
With the above definition. Our servlet reads the initialization parameters defined in the web. The program is very simple. a servlet can access the parameter in the init() method as shown below:
public void init(ServletConfig config) {
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.xml to it.xml is a good practice as it eliminates hard coding in the servlet. Initialization parameters to a servlet are defined in the web. in the future if the initialization values need to be changed.xml using the <init-param> element as shown below:
<init-param> <param-name>driver</param-name> </param-value>com.
Servlet Initialialization
Initializing a servlet is one of the most common practices. and displays them in the browser.Chapter 13
Servlet Programming
Servlet Initialialization
See how simple it is to write a servlet.jdbc. invokes the init() method of the servlet as part of the life cycle and passes the parameters defined in the web. The two parameters are driver and url for the MySQL database.mysql.xml without having to recompile the entire web application. Defining initialization parameters for servlets in web. All we did is followed a simple process.xml.

Following are the steps for processing a HTML form using a Servlet 1. Registration pages. The form data will then be sent to Servlet 3. processes it and send a confirmation. 2. Email composing etc are some of the examples. I think you might have noticed on various websites where you fill in html form with all the information and submit it for processing. Servlet reads the form data. Type the following URL in the browser http://localhost:8080/myweb/InitServlet The above URL displays the following
Reading HTML Form Data
HTML form processing is one of the most common things that any web application does.Chapter 13
Servlet Programming
Reading HTML Form Data
5. The form data will be sent in the HTTP request object as name value pairs as shown below:
http://localhost:8080/myweb/SomeServlet?name=John&age=20
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. User fills the data in the html page and submits it. The application then responds with a confirmation message something like “information is successfully processed”.

The data in this form will be submitted to a servlet using the following form element:
<form action="/myweb/FormProcessingServlet" method="GET">
The above form element sends the form data as a GET request to the FormProcessingServlet.*.getInitParameter("driver").3b (FormProcessingServlet. Now. drivername = config. import javax. PrintWriter pw = res. String databaseURL. HttpServletResponse res) throws ServletException.*. IOException { res. } public void doGet(HttpServletRequest req.http. public class FormProcessingServlet extends HttpServlet { String drivername.setContentType("text/html").3b. String fName = req. String mName = req.java) Servlet processing a form. Look at the code in listing 13.getParameter("middleName").
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. We do all the initialization here. let’s see how to write this servlet.
package myservlets.io. databaseURL = config. String lName = req. import java.getParameter("lastName").getWriter().servlet.getParameter("firstName"). public void init(ServletConfig config) throws ServletException { // This method is called before the following methods are called //and gets called only ONCE.getInitParameter("URL"). Listing 13.*. This is like a // constructor. // Read the form data String responseMsg = "". import javax.servlet.Chapter 13
Servlet Programming
Reading HTML Form Data
<td></td> <td><input type="submit" name="Submit" size=20 /> </tr> </table> </form> </body> </html>
The above code represents a simple HTML form will the customerm fields.

In the doGet() method.getParameter("age"). ssn = req. } } catch (Exception e) { responseMsg = "Error while Parsing Age and SSN.parseInt(ssn). int ssn. mName. it reads all the form data using the getParameter() method. } }
The FormProcessingServlet does the following things: 1. String country) { // Please write the JDBC logic to insert the data into the //Customers table. state = req. lName.getParameter("city"). Please try again". return 1. iage. country = req.. Make sure you enter numbers".
The getParameter() method takes the firstName as the key.parseInt(age). city. and returns
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.getParameter("ssn"). city = req. Take this as a home work. When the form is submitted. So lets //convert the strings to int and then store it to database int iage = 0. int age. try { iage = Integer. } else { responseMsg = "Failed to process the form. // Call the method that inserts into database int status = insertCustomer(fName. issn. String city. issn = Integer.getParameter("state"). age and ssn are integers.
// Remember in our database.country). String lastName. All the data will be returned as strings. state. } // Send the response pw. } private int insertCustomer(String firstName. Note: the driver name and the URL are //already available in the init() method. if (status != -1) { responseMsg = "Sucessfully processed the form".println("<h1" + responseMsg + "</h1>"). String middleName.getParameter("country").Chapter 13
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Reading HTML Form Data
String String String String String
age = req. int issn = 0. the URL will look as shown below:
http://localhost:8080/myweb/FormProcessIngServlet?firstName=Smith&…. // This method should return an int that the executeUpdate //methods returns. String state.

31>webapps>myweb>WEB-INF>classes>javac myservlet\*.html
2. Finally it sends the response message back to the browser.1.java
3.jdbc.java /myweb/htmls/CustomerForm. Type the following URL in the browser http://localhost:8080/myweb/htmls/CustomerForm.xml file
<!-Servlet Definitions -.>
<servlet> <servlet-name>FormProcessingServlet</servlet-name> <servlet-class>myservlets. Invokes the insertCustomer() method and passes all the data to it.Driver</param-value> </init-param> <init-param> <param-name>URL</param-name> <param-value>jdbc:mysql://localhost:3306/MDIT</param-value> </init-param> </servlet> <!-Servlet Mappings -. Take this as a home work.>
<servlet-mapping> <servlet-name>FormProcessingServlet</servlet-name> <url-pattern>/FormProcessingServlet</url-pattern> </servlet-mapping>
4. This method should insert the data into the database.mysql. Save the two files as
/myweb/WEB-INF/classes/myservlets/FormProcessingServlet.bat in the bin directory.html
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. 3. 2. Converts the age and ssn into integers.Chapter 13
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Reading HTML Form Data
Smith as the value. 5. Compile the servlet as shown below:
C:/>jakarta-tomcat-4. 4. Steps to run the Servlet 1. Start the server by clicking the startup. Add the following snippet in the web.FormProcessingServlet</servlet-class> <init-param> <param-name>driver</param-name> <param-value>com.

If you entered valid data. In web application terminology we call the shared data or information as state. Having state is just not sufficient. you’ll see the following message:
Error while Parsing Age and SSN. who does this state propagation? Can HTTP do this? No. it is important that one servlet shares information or data with other servlet. you need to go through multiple pages like search page. servlet A can create some state (information) and store it in the session. a typical web application comprises of several servlets that require collaborating with each other to give a complete response. A web container provides a common storage area called as session store the state and provides access to this session to all the servlets within the web application. if you go online to purchase a book. However. So. For instance. For instance. billing page etc before you can complete the transaction. the web container maintains a separate session for each and every user as shown in the following diagram. shopping page. because HTTP is a stateless protocol which means it cannot propagate the state. So.Chapter 13
Servlet Programming
Session Management
Fill in the form and click the submit button. there is one guy who is always there for our rescue and it’s none other than web container (Tomcat). is there anyone to help us out here to make the state available to all the servlets? Yes. Someone must be able to pass this state from one servlet to other servlet so that they can share the data. Since the state (data or information) in the session is user specific. Servlet B can then get hold of the session and read the state. In situations like this. you will see the message
Form Processed Sucessfully. Make sure you enter numbers
Session Management
The examples we saw until now just deal with one servlet.
If you entered characters for age or ssn.
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.

we do the following:
HttpSession session = request.
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.Chapter 13
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Session Management
Web Container (Tomcat)
State Servlet A Session Servlet B
User 1
User 2
State
Session Servlet C User 3 State
Session Fig 13. 1.2 Session Management
If you look at the above figure. Let’s see each of them one by one. 2. let’s see how a servlet uses the session for sharing the data across multiple pages in a web application. Also notice. 3. Create the session Store the data in the session Read the data from the session Destroy the session or invalidate the session. 4.
Creating a Session
The servlet API provides us with a class called HttpSession to work with sessions. Now that we know what a session is. To create a session. every user will have his own session object for storing the state pertaining to his transaction and all the servlets will have access to the same session.
The above four operations are called as Session Management operations.getSession(true). A servlet can do the following four most important operations to work with sessions. the session objects are present within the container.

45”)). it should forward the request to another servlet B.
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. we need to cast it again as Double while reading it.Chapter 13
Servlet Programming
Request Dispatching
The above method returns a new session object if one is not present.invaliadte(). otherwise it returns the old session object that is already created before.
Reading the data from the Session
To read the data.
Request Dispatching
Request dispatching is the ability of one servlet to dispatch or delegate the request to another servlet for processing. In simple words. A session is destroyed by invoking the invalidate() method as shown below:
session. To store the data we use the setAttribute() method as shown below:
session.
Notice that we need to do proper casting here. This servlet then does the rest of the processing and sends the final response back to the browser. Therefore. after partially processing the request.new Double(“12.
Destroying the Session
A session is usually destroyed by the last page or servlet in the web application. we need to use the getAttribute() method by passing in the key as shown below which then returns the value object:
Double d = (Double)session.getAttribute(“price”). let’s see a simple concept called Request Dispatching. Since we stored Double object. The value can be any Java object and the key is usually a String. let's say we have a servlet A which doesn't know how to completely process the request.
Storing the data in Session
Data in session is stored as key-value pair just like in HashMap or Hashtable.setAttribute(“price”.
Before we write an example using session management.

The forward() method This method is used for forwarding request from one servlet to another. The following figure shows the structure of the web application we are going to build. This method also takes HttpServletRequest and HttpServletResponse as parameters. one servlet can include the response of other servlet. Following two figures demonstrate dispatching using forward() and include() methods.
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. The include() method With this method. This method takes HttpServletRequest and HttpServletResponse as parameters. This interface has two methods namely forward() and include(). The first servlet will then send the combined response back to the browser. Using this method. Consider two servlets A and B.
Forward Dispatching Req A forwards Req A to
Servlet A
Servlet B
Res B
Req A
Servlet A
Res B
Servlet B
Res A + Res B Include Dispatching
Now that we also know something about request dispatching let’s build a mini web application using session management and request dispatching.Chapter 13
Servlet Programming
Request Dispatching
The class used for dispatching requests is the RequestDispatcher interface in Servlet API. A gets the request. which then forwards the request to B. B processes the request and sends the response to the browser.

html page will take the credit card information and sends the data to BillingServlet. 7.html.html to register 3.html LoginServlet login. It then forwards to ConfirmationServlet that displays the confirmation message with a thank you greeting.html page. If login fails.4a (login. 2.html ConfirmationServlet register. 5. The login. Upon checkout.3 A sample web application
Following is how the application works: 1.html OrderProcessingServlet billing.html page. It then forwards the request to billing. The user will shop the products in shop. Let’s build the components one by one.html RegistrationServlet BannerServlet BillingServlet
Fig 13. The response of BannerServlet will be included in the response of ConfirmationServlet. 6. shopping cart information will be sent to OrderProcessingServlet which processes the order and saves all the billing details in the session. The LoginServlet will process the login information.Chapter 13
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Request Dispatching
shop.html
Listing 13.html sends the login information to the LoginServlet. BillingServlet uses the billing data in the session and bills the customer.html) Login Form
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.html page. This servlet saves all the data in the database and sends him to the shop.html. The billing. it redirects the user back to login. it takes the user to shop. 8.html also sends the user to register.
login. Otherwise.html will take the customers information and sends it to RegistrationServlet. The register. 4. The login.

If not.bat in the bin directory. For now.
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. In reality. In the shopping page select the products and checkout. Type the following URL in the browser http://localhost:8080/myweb/htmls/login. This will send you to a billing page that takes the credit card details and gives you a confirmation message.html. 6. It passes the username and password to a method named isAuthenticated() which returns either true or false. Start the server by clicking the startup.>
<servlet-mapping> <servlet-name>LoginServlet</servlet-name> <url-pattern>/LoginServlet</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>RegistrationServlet</servlet-name> <url-pattern>/RegistrationServlet</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>OrderProcessingServlet</servlet-name> <url-pattern>/OrderProcessingServlet</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>BillingServlet</servlet-name> <url-pattern>/BillingServlet</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>ConfirmationServlet</servlet-name> <url-pattern>/ConfirmationServlet</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>BannerServlet</servlet-name> <url-pattern>/BannerServlet</url-pattern> </servlet-mapping>
5. then return false.Chapter 13
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</servlet>
<!--
Servlet Mappings
-.html Type in any username and password and you’ll enter in to the application. Following is how each of the Servlet classes work. you need to write database logic to verify whether the user exists in the database or not. LoginServlet This servlet reads the parameters posted by login. this method is defaulted to always return true.

html page.getRequestDispatcher("/htmls/login. // Store the data in the session session.
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. Following is the code that does this.getRequestDispatcher("/htmls/shop.html. rd.forward(req.forward(req. rd. It then stores the price in the session as shown below:
// Get the session HttpSession session = req.setAttribute("TotalPrice".html page.res).getSession(true). Otherwise. } else{ // Go back the login page RequestDispatcher rd = req.
if( isAuthenticated( userid. it uses the RequestDispatcher and forwards the request to the shop. which it does.new Double(totalPrice)).res). and computes the price of the order.html").html"). it forwards back to the login. password)){ // Go to the shopping page RequestDispatcher rd = req. }
OrderProcessingServlet This servlet reads all the data submitted by the form in the shop.Chapter 13
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Request Dispatching
If the method returns true.

forward(req.
Finally it forwards the request to ConfirmationServlet as shown below:
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. // Generate a random confirmation number long conf = (long)(Math. Finally it forwards the request to the billing page as shown below:
// Forward to Billing page req.html").new Long(conf)).setAttribute("Confirmation".html page and generates a confirmation number.Chapter 13
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Request Dispatching
The price will then used by the BillingServlet later on. It then stores this confirmation number in the session as shown below:
// Get the session HttpSession session = req. // Store the data in the session session.res).getRequestDispatcher("/htmls/billing.getSession(true).
BillingServlet This servlet reads the billing information posted by billing.random()*9999999).

getSession(true).
ConfirmationServlet This servlet reads both the price and confirmation number from the session. pw.doubleValue().07*price.Chapter 13
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Request Dispatching
// Forward to confirmation page req.longValue()+"<br/").forward(req. // Incude the response of Banner Servlet
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.getRequestDispatcher("/ConfirmationServlet").res).println("<h3> Your Order has been sucessfully processed</h3><br/>").doubleValue() + 0.getAttribute("TotalPrice"). and computes the overall price including the taxes (7%) as shown below:
// Get the session HttpSession session = req.
It then combines its own response message along with the response of BannerServlet as shown below:
pw.getAttribute("Confirmation"). // Compute the price including tax double totalPrice = price.println("<h3>The tracking Number for this order is "+ conf. pw. // Read the price and confirmation number Double price = (Double) session.println("<h3>Your credit card is charged for $ "+totalPrice+"</h3><br/"). Long conf = (Long) session.

we don't have to write a single println statement.
This completes all the important concepts in servlets.include(req. One of the primary drawbacks with servlets is the way they generate the html response. JSP makes our life very simple. All the examples we did until now are more than enough using Servlets. How will it be if we rather write Java code in a HTML file? Hmmm. This doesn't mean we can completely ignore Servlets.res). it’s tedious to write HTML markup in a Java program and I am sure you’ll agree with me.Chapter 13
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Request Dispatching
req. The underlying technology for both JSP as well as Struts is again Servlets. right? If this is the case. For a sec. we use whole lot of println() method to send the html response. So. but
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. This reverse technology is nothing but what we call as JSP technology. most of the web applications are moving away from using servlet technology and using JSP and open source technologies like Struts to build web applications. Practically. it’s always good to know how servlets work. This is really tedious and time consuming task.
Notice that we used the include() method as opposed to forward() method to include the response of BannerServlet. These days. let's try to reverse the situation here.getRequestDispatcher("/BannerServlet"). So all our Java code will be cluttered with these println statements which makes the servlet hard to understand and manage. The reason is simple. We just have to write the Java code in HTML file where ever needed. We will discuss this in the next chapter. sounds interesting. If you noticed. A typical web application will have 90% of HTML code and 10% dynamic content.

A typical web application will have several servlets that collaborate with each other.Chapter 13
Servlet Programming
Summary
again. all the query data is appended to the URI itself. To write a servlet we use Servlet API. If we do this.xml file is called as the deployment descriptor.
Summary
Web Applications use Internet for enterprise computing. Which of the following protocol is used by web applications
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. POST requests can send large amounts of data. The web container uses web.
Time to play 50-50
1. The most widely used HTTP requests are GET and POST. A Servlet is a server side program that runs in a web container to process the HTTP requests and send HTTP response. HTTP is a stateless protocol. In GET request. One of the main applications of servlet is form processing. a servlet uses the forward() method to dispatch the request to another servlet. In J2EE. The protocol used to send requests is HTTP. Servlets share the data using session. Servlets use RequestDispatcher object to delegate requests.xml to identify which servlets it need to load. The web container uses a separate session for every client. GET requests are using for sending smaller amounts of data that need not be secure. the underlying technology for JSP is Servlets. It also hides the data from being visible when sent to a server program. even the simplest things will get tough. The most popular one is the Tomcat container from Apache. The main difference between include and forward is that. I know you might be thinking why not jump right away to simplified technologies. In a web application. in the former a servlet uses the include() method to include the response of another servlet in it response while in the later. The beauty of J2EE is that it simplifies life as we move forward. The web. let’s move step by step. So. a browser sends requests to server side programs. A session is created and maintained by the web container. learning Servlets is worth the time. So. the server side programs run in a Web Container. The server side programs processes the requests and send the responses.

Which of the following is a method in RequestDispatcher to add the response of another servlet a) forward b) include
Interview Questions
Question: What is a Servlet?
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. Which of the following method is used by a servlet to delegate a request? a) forward() b) dispatch() 7. Which of the following is used to read the data in a HTTP request a) HTTPServletRequest b) HttpServletRequest 3. Which of the following file is used as a deployment descriptor for web applications? a) servlet.xml 4.xml b) web. Which of the following is used for managing sessions in a Servlet? a) HttpSession b) HtmlSession 5.Chapter 13
Servlet Programming
Interview Questions
a) HTML b) HTTP 2. Which of the following class is used to dispatch requests? a) RequestDispatcher b) ServletDispatcher 6.

Question: What is the difference between include and forward dispatching? Answer: Refer to the Request Dispatch section
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. the browser sends the SESSIONID back to the server which it uses to identify the client. In this approach. Question: What is the name of the deployment descriptor a web application uses? Answer: web. Hidden Form fields and URL rewriting Question: What is URL rewriting? Answer: URL rewriting is a technique used for session management.Interview Questions Servlet Programming
Answer: Server is a server side Java program that runs in a web container for processing HTTP requests and sending HTTP responses.xml Question: What are the various ways of managing sessions? Answer: Cookies. a web container generates a unique identifier called as SESSIONID and appends to every URL in the page. When the URL is clicked.

By the end of this chapter.. Create a directory named beans in the following directory in Tomcat
C:/…./…/webapps/myweb/WEB-INF/classes/customtags
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./…/webapps/myweb/WEB-INF/classes/beans
3./…/webapps/myweb/jsps
2.. Create a directory named jsps in the following directory in Tomcat
C:/…. you’ll become familiar in building web applications using JSP pages. Create a directory named customtags in the following directory in Tomcat
C:/….Chapter 14
JSP Programming
Chapter Goals
Chapter 14
JSP Programming
This chapter introduces you to the most widely used web technology. JSP technology is the coolest and simplest of all web technologies and I am sure you’ll enjoy reading it. the Java Server Pages (JSP).
Chapter Goals
Understand the fundamentals of JSP Understand the JSP directives Understand JSP scriptlets Using Implicit Objects Using Java Beans Custom Tag Anatomy Building custom tags
Environment Setup
1..

This technology is built to overcome all the cons of Servlet technology.jsp” extension. HTML + Java = JSP Therefore. more is the success it tastes. let me ask you a quick question. abstraction is a simplified fine grained layer over a slightly more complex layer that makes the development faster and easier. let’s jump into the real world of JSP and see how it justifies all the good things about it that we stated thus far. What does abstraction mean? In simple terms. Good. in essence. No we can’t. Sun Microsystems decided to build a technology that completely simplifies the development of web applications by embedding Java code inside html markup as opposed to html markup inside Java code used by servlets. To summarize. With this brief introduction. present and will be the future technology for any web application built using Java and J2EE.Chapter 14
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Introduction
Introduction
JSP is yet another J2EE technology for building web applications using Java. If this is the case. This is applicable to everything.html” extension? You guessed it right. keep your life as simple as possibly can. Since most of the web pages have static content with little dynamic content. This is the reason why we are seriously interested to learn and digest this cool technology. we need to use a different extension which is the “. Let me tell you one thing here. Can we save this mixed content in a file with “.
JSP Basics
A typical JSP page very much looks like html page with all the html markup except that you also see Java code in it. The simpler a technology gets. JSP is the past. with a mixture of HTML and Java. a html
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. not just Java and J2EE. In a single sentence. So. one of which is the way it renders the html response. JSP technology has become an instant hit and has become the de facto standard for building web applications using Java. We want the Java code to be executed and display the dynamic content in the page. JSP content is a mixed content. because the html formatter will also treat the Java code as plain text which is not what we want. This is why we can call JSP as an abstraction over servlet. and you’ll truly taste success. more the simplicity. One thing you must know at this point is that JSP technology is built on top of servlet technology. For this to happen. More the abstraction. Trust me. One of the primary reasons for the success of any technology is the simplicity and ease that it offers.

The servlet then processes the Java code. Now. and a jsp file will have both html markup and Java code. The web container instead of scratching its head to identify Java code. look at the following figure:
HTML request
B R O W S E R
Web Container (Tomcat) HTML response
JSP request
Web Container (Tomcat)
Translates to
Servlet
HTML response
Fig 14. when the browser sends a JSP page request. we need to know how it does this translation. Let’s understand what these special symbols are. Don’t think anything beyond this. Though the web container does the translation of JSP to Servlet. simply uses the special symbols to identify it. I am sure the picture is now 100% clear. Following table shows these placeholders along with how the web container translates the Java code with them. the web container simply responds with a html response without any processing. and translates the page into a Servlet. However. According to J2EE specification. a JSP page must use special symbols as placeholders for Java code. and returns the complete html response including the dynamic content generated by the Java code. This is a contract between JSP and the Web container. For a web container to translate JSP. When the browser requests for html file. the web container assumes that the JSP page might include Java code. This is an example of making life easy. as to which is HTML markup and which is Java code.Chapter 14
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JSP Basics
file will only have html markup.1 HTML and JSP Requests
The above picture should give you some idea of how a web container processes html requests and JSP requests. In JSP. it needs to identify from the JSP page.
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. Point to be noted. we use four different types of placeholders for Java code.

then its uses page directive attribute as shown below:
<%@ page session=”true” %>
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. There are 3 types of directives as listed below: page directive include directive taglib directive The page directive This directive is used to declare things that are important to the entire JSP page. The syntax for this directive is shown below <%@ page attribute list %> Following table lists the most widely used attributes with this directive: Table 14.Chapter 14
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JSP Basics
developer is.*. how and for what purposes we use various placeholders..2 Page directive attributes page Attribute import session errorPage isErrorPage Description Used by the current JSP for importing the resources Used by the current JSP for session management Used to specify the error pages for the current JSP Used to specify the current JSP as an error page to some other JSP
Ex 1: If the JSP page needs to import the core library classes. java.io. and does not produce any response to the client. it uses the page directive as:
<%@ page import=”java. So.
JSP Directives
Directives are used for declaring classes.*” %>
All the resources must be separated by a comma as shown above. Ex 2: If the JSP page needs to use the session.util. let’s see what these are. A directive uses attributes for declarations. Understanding this hardly takes few minutes. session usage etc. without wasting any further time.

we can define all the attributes at a time as shown below:
<%@ page import=”java.jsp) JSP using page directive
<%@ page import="java. and type the following URL in the browser
http://localhost:8080/myweb/jsps/PageDirectiveDemo.jsp”. Ex 3: If a JSP page should forward to a custom error page for any exceptions within the page. So. it should use the following page attribute:
<%@ page isErrorPage=”true” %>
Instead of defining one attribute per page directive.jsp
Since we are writing Java code in JSP.*” session=”true” errorPage=”Error. so take a look at the code in listing 14.1 (PageDirectiveDemo.*" session=”true” isErrorPage=”false”%> <HTML> <BODY> <h4>Welcome to the world of JSP</h4> This JSP uses the page directive </BODY> </HTML>
Steps to run the JSP 1.util.jsp if it encounters exceptions within the page.Chapter 14
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JSP Basics
If the session attribute is set to true. Such pages really freak the customers
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.util.jsp” %>
The web container will then forward the request to Error. there is 100% chance that the Java code may throw an exception. 2. Ex 4: If a JSP page should allow itself as an error page for other JSP pages. Save the code as “/myweb/jsps/PageDirectiveDemo. Start the server. what happens when the Java code in JSP throws some exception? You’ll see the very familiar page not found error.jsp” %>
I know you are waiting to see an example.1 that uses a page directive. the page directive will be as shown below:
<%@ page errorPage=”Error. then the page will have access to session. Listing 14.

This is like mutual understanding between the pages. Good web applications usually display custom error page rather than blank pages.
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. Start the server. It declares the page directive attribute errorPage to use Error.2b (Error.jsp page grants permission to use it as an error page by using the page directive attribute isErrorpage with the value set to true. Please don't panic and call 1-800-888-9999 for help desk </h3>
Steps to run the JSP 1.jsp" %> <% int i=0.jsp as an error page incase any exception pops up. we can let the users know what might have caused the error and give them a help desk number for customer support. int k = 10/i. then B should grant the permission to A.jsp.jsp
2. The Error.jsp) JSP using page directive
<%@ page isErrorPage="true" %> <h3> An exception is thrown by the page you're trying to access. JSP uses the page directive attribute errorPage.jsp and /myweb/jsps/Error.jsp) JSP using page directive
<%@ page errorPage="Error.jsp
Look at the ErrorPageDemo. We’ll see about it later. Listing 14. By displaying error pages. and type the following URL in the browser:
http://localhost:8080/myweb/jsps/PageDirectiveDemo.2. This is exactly what both the above JSP’s are doing. Save the above two files as
/myweb/jsps/ErrorPageDemo. Let’s first write a piece of code to demonstrate its usage and then see how it works. To display custom error pages.2a (ErrorPageDemo. // This throws Arithmetic Exception %>
For now. don’t worry about the code in <% and %> symbols.Chapter 14
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JSP Basics
who visit the web site. Listing 14. If A needs to use B. See the code in listing 14.

Chapter 14
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JSP Basics
When the above URL accesses the ErrorPageDemo. it throws ArithmeticException due to division by zero.3 (IncludeDirectiveDemo. Its usage is shown below. Save the code as
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. <%@ include file=”file name” %> See the code in listing 14.jsp.
The include directive This directive is used to include the response of another resource (JSP or html) at the point where the directive is placed in the current JSP.jsp which displays the custom error message. Therefore. Following is what you’ll see in the browser when you access the page.jsp) JSP using include directive
<HTML> <BODY> <h4> This is the response from the current JSP page</h4> <h3> Following is the response from another JSP </h3> <hr/> <%@ include file=”/jsps/PageDirectiveDemo.3. Listing 14. the web container gracefully forwards it to the linked error page Error.jsp” %> <hr/> </BODY> </HTML>
Steps to run the JSP 1.

don’t worry about it. For the current JSP to use a custom tag. Start the server.
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. Custom tag definitions are usually defined in a separate file called as Tag Library Descriptor. let’s see the usage of another type of placeholder. For now. and type the following URL in the browser
http://localhost:8080/myweb/jsps/IncludeDirectiveDemo. <%@ taglib uri=”location of definition file” prefix=”prefix name” %> Custom tags and tag libraries are explained in detail in the later pages and we’ll see how to use this directive at that point.Chapter 14
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/myweb/jsps/IncludeDirectiveDemo. it needs to import the tag library file which is why this directive is used. Following is how taglib directive is used.jsp
The above URL will produce the following output:
The taglib directive This directive allows the JSP page to use custom tags written in Java. Now that we know how and when to use JSP directives.jsp
2.

Date" %> <%! String getGreeting( String name){ Date d = new Date().jsp) JSP Declations
<%@ page import = "java.jsp
The above URL will produce the following output:
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. Listing 14. The JSP invokes the global method to produce the following </h3> <hr/> <h3> <%= getGreeting("James Bond") %> <hr/>
Steps to run the JSP 1.jsp
2. A declaration block is enclosed within <%! and %> symbols as shown below: <%! Variable declarations Global methods %> Take a look at the code in listing 14.4 (JSPDeclarationDemo. } %> <h3> This is a JSP Declaration demo. return "Hello " + name + "! It's "+ d + " and how are you doing today". Start the server. Save the code as
/myweb/jsps/JSPDeclarationDemo. and type the following URL in the browser:
http://localhost:8080/myweb/jsps/JSPDeclarationDemo.Chapter 14
JSP Programming
JSP Basics
JSP Declarations
JSP declarations are used to declare global variables and methods that can be used in the entire JSP page.util.4.

jsp) JSP expressions
<%! String name="John Smith". An expression could be a variable or a method that returns some data or anything that returns a value back. String address = "1111 S St.Thank you for shopping at BuyForLess. Expressions are enclosed in <%= and %> as shown below <%= Java Expression %> Take a look at the code in listing 14. Lincoln.com". USA". } %> <h3> Your order is sucessfully processed.Chapter 14
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JSP Basics
JSP Expressions
Expressions in JSP are used to display the dynamic content in the page.5 (JSPExpressionDemo. NE. The confirmation number is 876876
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.5 Example 4: Look at the following JSP code Listing 14. String getMessage(){ return "Your shipment has been sent to the following address.

Save the code as
/myweb/jsps/JSPExpressionDemo.Chapter 14
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JSP Basics
</h3> <hr/> <h3> <%= getMessage() %> </h3> <h4> <%= name %> <%= address %> </h4> <hr/>
Steps to run the JSP 1. Start the server. Scriptlets are enclosed
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. and type the following URL in the browser:
http://localhost:8080/myweb/jsps/JSPDeclarationDemo.jsp
The above URL will produce the following output:
JSP Scriptlets
A Scriptlet is a piece of Java code that represents processing logic to generate and display the dynamic content where ever needed in the page.jsp
2.

jsp
The above URL will produce the following output:
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. Look at the code in listing 14. Save the code as
/myweb/jsps/JSPScriptletDemo.i++){ if( i % 2 == 0){ %> <br/><%=i %> is an even number <% %> <br/><%= i %> is an odd number <% } } // End of for loop %> </HTML> } else{
Steps to run the JSP 1.6 (JSPScriptletDemo. Start the server.Chapter 14
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JSP Basics
between <% and %> symbols. This is the most commonly used placeholder for Java code.Methods</TITLE></HEAD> <h3> This is an example using Scriplets </h3> <% int sum=0.i++) { sum+=i.jsp) JSP scriptlets
<HTML> <HEAD><TITLE>Tag . and type the following URL in the browser:
http://localhost:8080/myweb/jsps/JSPScriptletDemo.i<=100.jsp
2.6. }
%> <hr/> The sum of first 100 numbers is <%= sum %> <hr/> <h3> Following is generated by the Loop <% for(int i=2. for(int i=1. Listing 14.i<=5.

Let me tell you a small trick here. This makes life easy. Write Java code and HTML without using <% and %>. Remember we processed HTML forms using servlet? Let’s see how JSP handles this aspect of web applications. 1. follow the following two steps. Following table lists the implicit objects. let’s look at something called Implicit Objects. 2.3 JSP Implicit Objects Implicit Object request response Description This is an object of HttpServletRequest class.
Table 14.
Implicit Objects
As the name suggests every JSP page has some implicit objects that it can use without even declaring them. All the concepts we learned so far are very simple. You can use them as you wish to build complex web pages. right? This is why JSP is the coolest web technology. Used for
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.Chapter 14
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JSP Basics
The above four symbols are all the Java placeholders that we use in a typical JSP page. Used for reading request parameters (Widely used) This is an object of HttpServletResponse class. Before we get to this. Once you finished writing the scriptlet code. When you write a scriptlet. The most important one is the JSP scriptlet. then divide the Java code and html code using <% and %>. The JSP page can readily use them for several purposes.

7 (Login.jsp"> <table border=1> <tr> <td> Login Name </td> <td> <input type="text" name="username" size="20"/></td> </tr> <tr> <td> Password </td> <td> <input type="password" name="password" size="20"/></td> </tr> <tr> <td> <input type="submit" value="Submit"/></td> </tr> </table> </form> </html>
Listing 14. In this example. only request and session objects are widely used in JSP pages. This is an object of JspWriter.
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. The request object is used for reading form data and session object is used for storing and retrieving data from session.7b (StoreData. Similar to PrintWriter in Servlet. Rarely used. Rarely used.
Out of all the above implicit objects. So. This is an object of HttpSession class used for session management. Take a look at the code in listing 14.getParameter("username"). (Widely Used) This is an object of ServletContext. Used to share data by all Web applications.jsp) JSP that stores the form data in session
<%@ page session="true" %> <% // Read the data from the request String name = request. don’t worry about it. This is almost never used by the JSP pages. We will then write another JSP that reads the data from the session and displays them in the browser.jsp) Simple login page
<html> <body> <form action="StoreData.7 Listing 14. we will have html form post some data to a JSP page with reads the form parameters and stores them in the session. Let’s see an example using request and session implicit objects.Chapter 14
JSP Programming
JSP Basics
session application out
displaying the response content.

Chapter 14
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JSP Basics
String pwd =
request. %> <h3> The username is <%= name %> <br/> The password is <%= pwd %>
Look at the code in Login. session.jsp">Click Here</a> to go to the page that displays the data.name).getParameter("password").
session. Read the form data using the request implicit object.jsp does two things listed below: 1. This page simply has a html form that posts the data to StoreData.
This page then provides a link to the RetriveData. %> <h3> This page read the form data and stored it in the session.pwd).jsp) JSP that reads the form data in session
<%@ page session="true" %> <% // Read the data in the session String name = (String)session.7c (ReadData.getParameter("password").
String name = request.setAttribute( "userid".jsp.</h3> <a href="RetrieveData.getParameter("username").name). session.setAttribute("password".
2.jsp”>
The StoreData. String pwd = request. The RetrivePage.setAttribute("userid".jsp as shown below:
<form action=”StoreData.
Listing 14. Store the form data in the session using the session implicit object.getAttribute("password").
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. String pwd = (String)session.pwd).getAttribute( "userid").setAttribute("password".
// Store the data in the session session.jsp.jsp reads the data stored in the session as shown below and displays it.

jsp
The above URL will produce the following output:
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.jsp
2. Start the server.jsp /myweb/jsps/SaveData. Save the code as
/myweb/jsps/Login.
Steps to run the JSP 1. and type the following URL in the browser:
http://localhost:8080/myweb/jsps/Login. String pwd = (String)session.jsp /myweb/jsps/ReadData.getAttribute("password").getAttribute( "userid").Chapter 14
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JSP Basics
String name = (String)session.

Following are the three standard actions for working with Java beans: 1. JSP technology uses standard actions for working with beans. let’s see how we can use beans in JSP and simplify our life. <jsp:getProperty> Let’s see the above standard actions one by one. <jsp:setProperty> 3. So. where should the business logic be? Good question. If this is the case. A typical JSP page should have as minimum business logic as possibly can. The business logic will be moved into external entities which will then be accessed from within JSP page.
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. A Java bean as we learned at the beginning of the book is a simple class with getters and setters. These external entities are nothing but Java beans.
Java Beans in JSP
One of the good practices while writing JSP is to isolate the presentation logic (HTML) from business logic (Java code).Chapter 14
JSP Programming
Java Beans in JSP
All the examples we’ve seen until now demonstrate some of the basic capabilities of JSP. Using Java beans in JSP offers whole lot of flexibility and avoids duplicate business logic. <jsp:useBean> 2. The real power of JSP is its ability to use Java beans and the properties of the beans. Let’s see what this is all about.

If it finds one.Fully qualified name of the bean class scope – The attribute that defines where the bean will be stored or retrieved from. 2.
jsp:setProperty
This action as the name suggests is used to populate the bean properties in the specified scope. If the scope is request.
<jsp:setProperty name ="bean name" property ="property name" value= "data" />
For instance. it returns the bean to the JSP. If the bean is not found. stores it in the session and returns it to the JSP. The syntax for this action is shown below: <jsp:useBean id=”bean name” class=”class name” scope=”scope name”/> where. 1. Following is the syntax for this action. Tries to locate the bean with the name cus in session scope.Customer” scope=”session” />
With the above declaration. id .A unique identifier that references the instance of the bean class . If the scope is session.Chapter 14
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jsp:useBean
This action is used by the web container to instantiate a Java Bean or locate an existing bean. The web container then assigns the bean to an id which the JSP can use to work with it. following is what the web container does. then the bean will be available to all the requests made by the client in the same session.
<jsp:useBean id = “cus” class=”beans. can be request or session (widely used) Consider the following declaration. The Java Bean is usually stored or retrieved in and from the specified scope. then the bean will only be available for the current request only. if we need to populate a bean whose property is firstName with a value John we use this action as shown below:
<jsp:setProperty name "cus" property ="firstName" value= "John" />
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. then the container instantiates a new bean.

<jsp:getProperty name=”cus” property=”firstName” scope=”session” />
Don’t worry even if you are confused with the above actions. It then provides a link to a servlet which retrieves the bean from the session and process the data in it.1 depicts this example. Fig 14.
htm l JSP Servlet
Customer Bean
Fig 14. while in scenario 2. The JSP will then retrieve the bean and displays the information to the client. Scenario 1: JSP collects the data from the client. In scenario 1. Following is how we can use this action to retrieve the value of firstName property in a bean identified by cus in the session scope.8 Listing 14. Scenario 2: A Server side component loads a Java Bean with all the information and stores it in the request or session.Chapter 14
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jsp:getProperty
This standard action is used to retrieve a specified property from a bean in a specified scope. The JSP page will then collect the data and store it in a Java Bean named Customer in session scope. The following example demonstrates scenario 1 in which a html form posts the data to a JSP page.8a (CustomerData. populate the bean's properties and stores the bean in request or session scope. This bean will then be used by another server side component to process the data. A quick example will clear all the confusions. JSP uses the bean to collect the data into it. it uses the bean to read the data from it to display.html) Simple html form that posts info to a JSP
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. look at the code in listing 14. There are two common scenarios with using JavaBeans in JSP.1 Scenario 1 of beans usage
Based on the above figure.

int ssn.html will post all the customer information to the CustomerInfoGatherer. the web container invokes the setFirstName() method and stores the data. the class attribute should be beans.
Following is how the above code works: The form in the CustomerData.Customer. At this point the
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.jsp after declaring the bean.
pw. } private int insertCustomer(String firstName. Since the class is stored in beans package. Take this as a home work. String state. will populate all the bean properties from the request parameters as shown below:
<jsp:setProperty name="userInfo" property="firstName" value="<%= request. int age. This JSP will first create the Customer bean using the following action:
<jsp:useBean id="userInfo" class="beans.getParameter("firstName") %>" />
The above action will populate the firstName property in the bean with the value in the request parameter came from the html form. Let me ask you a question here. The class attribute must specify the fully qualified name of the bean class. String middleName. This method should return an int that the executeUpdate methods returns.
return 1. String country) { // // // // // } } Please write the JDBC logic to insert the data into the Customers table. The CustomerInfoGatherer.println("<br/><h3>" + responseMsg + "</h3>"). String lastName. we populate the rest of the bean properties. This is why we need to define the setters in the bean class to set the property values. How does the web container use the above action and populate the actual bean property? Good question. Similarly.Customer" scope="session" />
The above action creates a customer bean with the id userInfo and stores it in the session. Internally. The customer bean simply defines all the fields (properties) along with the getters and setters for every property. Note: the driver name and the URL are already available in the init()method. String city.jsp.Chapter 14
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" "
"+ ssn + " " + city + " + state + " " + country).

>
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.getSession(true).CustomerInfoProcessor</servlet-class> <init-param> <param-name>driver</param-name> <param-value>com.
Notice that the session attribute name must be userInfo. Save all the Java files in the following directories
/myweb/WEB-INF/classes/beans/Customer. I am sure you can understand what we did. Add the following snippet in the web. Save the html files in the following directory
/myweb/htmls/CustomerData.mysql.jdbc. the servlet simply invokes the bean getters and get the bean data. Since the object in the session belongs to Customer class. Steps to run the JSP 1... The JSP page will then provide a link to a servlet that processes the bean.>.Chapter 14
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customer bean is fully populated with the data and is available in the session.java
3.. the web container invokes the doGet() method of the CustomerInfoProcessor servlet.xml file
<!-Servlet Definitions -.jsp
2.>.>classes>javac beans\*.java /myweb/WEB-INF/classes/myservlets/CustomerInfoProcessor. Also notice the casting we did.Driver</param-value> </init-param> <init-param> <param-name>URL</param-name> <param-value>jdbc:mysql://localhost:3306/MDIT</param-value> </init-param> </servlet> <!-Servlet Mappings -. When the hyperlink is clicked. since this is the name the JSP defined for the bean.html /myweb/jsps/CustomerInfoGatherer. From here onwards..getAttribute("userInfo").java c:/>JavaTraining>. Compile the classes as shown below:
c:/>JavaTraining>.>
<servlet> <servlet-name>CustomerInfoProcessor</servlet-name> <servlet-class>myservlets. This servlet first reads the customer bean from the session as shown below:
Customer customer = (Customer)req.java
4.>classes>javac myservlets\*. we need to cast it as shown. Once the bean is retrieved from the session.

1.31>webapps>myweb>WEB-INF>classes>startup
6.
C:/>jakarta-tomcat-4.html
The above URL will produces the following result.Chapter 14
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<servlet-mapping> <servlet-name>CustomerInfoProcessor</servlet-name> <url-pattern>/CustomerInfoProcessor</url-pattern> </servlet-mapping>
5.
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. Start the server using the following command and wait for few seconds. Type the following URL in the browser
http://localhost:8080/myweb/htmls/CustomerData.

jsp. right? What if my html form has 100 fields? Should I write 100 setProperty actions in my JSP? No if you follow a simple trick. Use the following one and only one setProperty action with the property attribute set to asterisk(*) to populate all the form data in all the bean properties in one step as shown below:
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size=100/>
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For a second. The bulk of the code is occupied with the setProperty actions. The trick is: 1. Make all the html forms field names same as bean property names as shown below: In the html file : <input type=”text” name=”firstName” In the Customer bean class: String firstName. go back and observe the code in CustomerInfoGatherer.

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. we already implemented Step 1 (matching names and properties).Customer" scope="session" /> <jsp:setProperty name="userInfo" property="*" <H3>Finished storing in the seesion </H3> <br/> <a href="/myweb/CustomerInfoProcessor" >Click Here </a> to invoke the servlet that process the bean data in session.jsp) Updated JSP
<HTML> <HEAD> <TITLE></TITLE> </HEAD> <BODY> <H3>Readingthe form data</H3> <H3>Polulating the bean and Storing in session </H3> <jsp:useBean id="userInfo" class="beans.jsp will now look as shown in listing 14. Listing 14. So. Fig 14. and you’ll notice the same result. will create.2 depicts this. The servlet then forwards the request to a JSP page which retrieves the bean from the session and displays it to the client. In this scenario.8e (CustomerInfoGatherer. we will have a JSP that sends customer name to a servlet. Isn’t this cool? I am sure it is. </BODY> </HTML> />
Run the example again. The servlet based on the name. In this example. let’s remove all the setProperty actions and replace all of them with the above setproperty action.8e.Chapter 14
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<jsp:setProperty name="userInfo" property="*" />
That’s it. This is why JSP is really a cool technology. populate and store the bean in the session. Let’s now look at scenario 2 of JavaBeans application. All the form data will be stored in the bean in one go. The updated CustomerInfoGatherer.

Driver</param-value> </init-param>
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appropriate customer object based on name. Save the html files in the following directory /myweb/jsps/Search.jsp to display the search results. If you look at the SearchResult. it forwards the request to a SearchResult.. it uses the getProperty actions as shown below to display the properties of the bean.jsp /myweb/jsps/SearchResult. Add the following snippet in the web..
Finally.customer). The servlet then stores the returned customer object with the name searchresult in the session as shown below:
req.>.Customer" scope="session" />
Notice that the id must be the same name that the servlet used to store the bean in the session.xml file
<!-Servlet Definitions -.jsp page. the scope attribute must be set to session as this is where our servlet stored the bean.getSession(true).setAttribute("searchresult".jdbc.mysql.java
4. SearchServlet </servlet-class> <init-param> <param-name>driver</param-name> <param-value>com. Save all the Java files in the following directories
/myweb/WEB-INF/classes/myservlets/SearchServlet. it first retrieves the customer bean from the session as shown below:
<jsp:useBean id="searchresult" class="beans.
<tr> <td> First Name </td> <td><jsp:getProperty name="searchresult" property="firstName" </tr> /></td>
Steps to run the JSP 1.>
<servlet> <servlet-name>SearchServlet</servlet-name> <servlet-class>myservlets.>classes>javac myservlets\*.jsp 2. Once it gets the bean.java
3. Also. Compile the classes as shown below:
c:/>JavaTraining>.

for(int i=1. Such beans are one side of the coin. A JSP is fundamentally a presentation component which should have presentation logic instead of business logic. the JSP uses a scriptlet to compute the sum of 100 numbers. Let’s write an example with and without using the beans and you’ll notice the beauty of beans.
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. See the code in listing 14. } {
%> <h3> The sum of first 100 numbers is <%= sum %> </h3> </HTML>
If you look at the above code. To demonstrate this. See listing 14. All this is because of the presence of the Java code in the JSP.10a which doesn’t use the beans. If someone comes and gaze though the page. he will simply be stumped because he doesn’t know what the heck is going on in the page. Listing 14.Chapter 14
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In the previous two examples if you notice the customer bean class.
<HTML> <% int sum=0.i++) sum+=i.10a (BadCalculator. all it has is getters and setters for storing and retrieving the data. which is good.jsp) Simple JSP without beans. Let’s see how we can improve the code using beans. let’s look at an example. This is where the real power of JavaBeans is exposed. The above code suffers from one serious drawback not in terms of functionality but in terms readability of the code.10b which uses beans. The other side of the coin is the beans that encapsulate business logic.i<=100.

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.10b (GoodCalculator.Chapter 14
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computing logic is now moved from JSP page to the bean method.
<HTML> <body> <jsp:useBean id="calculator" class="beans. Start the server and type the following URL
http://localhost:8080/myweb/jsps/GoodCalculator. it can simply do so as shown below: Listing 14.jsp) Simple JSP with beans. if the JSP need to compute the sum of other numbers.jsp code again. Notice the GoodCalculator. right? This is another beauty of using beans. It eliminated duplicate business logic and at the same time improved the readability of code several times. The good thing with this implementation is that.jsp
The above URL produces the following result:
This completes most of the important things you need to know about using JavaBeans in JSP pages.Calculator" /> <jsp:setProperty name="calculator" property="count" value="100" /> The sum of first <jsp:getProperty name="calculator" property="count" /> numbers is <jsp:getProperty name="calculator" property="sum" /> <jsp:setProperty name="calculator" property="count" value="150" /> The sum of first <jsp:getProperty name="calculator" property="count" /> numbers is <jsp:getProperty name="calculator" property="sum" /> </body> </HTML>
If you noticed the above JSP code. we completely avoided duplicate business logic.

The sum of first <jsp:getProperty name="calculator" property="count" /> numbers is <jsp:getProperty name="calculator" property="sum" />
We eliminated duplicate Java code using JavaBeans. A custom tag like any standard html tag will have the following things: 1. Let’s see what custom tags are and how they aid in the development of simplified JSP pages. To write a custom tag and use it in a JSP page. 2. The notion of custom tag is such a big hit that you literally see them in almost every single JSP page in any real world J2EE based web application. which looks just like an XML tag in terms of representation. A tag name One or more tag attributes Body Content Nested tags
Following is how a typical custom tag looks like.
Custom Tags
Custom Tags are introduced from JSP 1. A custom tag is a user defined tag. 4.
<bean:write name=”some name” property=”some prop”> This is the body content </bean:write>
In the above custom tag. It’s nothing but using custom tags. a careful insight into the code reveals another problem. This is a very small price compared to the benefits it offers. Now you might be wondering if there is another solution to also eliminate the duplicate html content. Failing to do so will result in a JSP translation error. Custom tags are extremely powerful and offers a whole lot of flexibility. write is the name of the tag and name and property are the attributes of the tag. So. we need to follow simple process. without wasting any further time. but conveys a special meaning to the web container when it comes across it. With custom tags it’s very important that every tag that is opened must also be closed. then our JSP will be a killer and you know what.Chapter 14
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However.
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. 3. let’s get into the details of custom tag anatomy. we have a solution. This is the last concept in this chapter. Are we greedy here? If we do have one. This is duplicate HTML code as shown below highlighted in bold.1 specification.

Declare one instance variable per attribute of the tag. Write a class that extends TagSupport class.JspException. Using the above process. 1.11a that’s shown a tag class for the above tag.
Just remember the above process even if you don’t understand the custom tags. we need to write a Java class using the standard tag extension API.TagSupport.java) A simple tag class
package customtags. it’s very easy to write this class.Chapter 14
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Following is the standard process for writing a custom tag. 2. This API has few built-in classes that we need to use to write the Tag class.servlet. Step 1: Writing a Tag class A custom tag can be viewed as a “representative” of some Java code in a JSP page. import java.io. To write a basic custom tag. 3. import javax. use the tag.jsp. You’ll be good. 4.tagext. Write a Tag class Define the tag class in a tag library descriptor Import the tag library descriptor into JSP page Finally. 3.jsp. let’s write the following custom tag that displays the sum of numbers. and define a getter and setter method. 2.IOException.
<calc:sum count=”100”/>
The web container should compile the above tag and produce the following html markup.
<h3>The Sum of 100 numbers is 5050</h3>
See listing 14. To write a custom tag. 1.servlet. import javax. Let’s now see the details of above steps one by one. This is our first goal.11a (CalculatorTag. Trust me. Overwrite two methods namely doStartTag() and doEndTag(). Listing 14.
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.getSum(). } // This return type tells the JSP page to continue processing // the rest of the page return EVAL_PAGE. Since our custom tag takes count as an attribute. String sum = calc. } }
If you observe the above code.write( "The Sum of first " + count + " numbers is " + sum). we created a class named CalculatorTag that inherits from TagSupport class. return EVAL_BODY_INCLUDE.count = count. public class CalculatorTag extends TagSupport { // This method will be called when the JSP encounters the start of the // tag implemented by this class. } public void setCount(String count) { this.*.getOut(). we defined a property (instance variable) named count and implemented both the getter and setter method (Rule 2). public String getCount() { return count. We then overwrote two methods namely doStartTag() and doEndTag() (Rule 3). } catch (IOException e) { throw new JspException( "Fatal Error: HelloTag could'nt write to the JSP out").Chapter 14
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import beans. Calculator calc = new Calculator(). String count. try { pageContext. } public int doStartTag() throws JspException { // This means the JSP must evaluate the contents of any CHild tags // in this tag.setCount(count). } // This method is called when the JSP encounters the end of te tag // implemented by this class public int doEndTag() throws JspException { // Use the bean that we already have to calculate the sum. calc. a built-in class that supports custom tags development (Rule 1).

Save the above file in the following directory
/myweb/WEB-INF/classes/customtags/CalculatorTag. This is where we need to do what ever we want to have the custom tag render the desired html to JSP page. Attributes of the tag
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. it first uses the Calculator bean class. This is the most important step.Chapter 14
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When the web container comes across the custom tag in the JSP. it need to use the pageContext object inherited from the parent class and use the write() method as shown below:
pageContext.If you notice the implementation of this method. so please go ahead with the rest of the page”. the web container is at the end of the tag. I am done processing the tag.
Once the sum is computed. the web container gracefully goes to the next step which is the invocation of doEndTag() method.getSum(). Where did this come from? Don’t worry. For the custom tag to return anything to the JSP. We finished writing the tag class. Returning this constant means. it should return it back to the JSP in proper format.
Now that it finished sending the desired html markup to the JSP. calc. That’s it guys. This means. A tag library descriptor is a standard XML file that defines all the details of the custom tag that the web container needs to process it. Class of the tag 3. it must return the constant EVAL_PAGE. It then calls the doStartTag() method.getOut().write("<h3>The Sum of first "+count+" numbers is " + sum + “</h3>”). otherwise go ahead with the next step”. it follows a systematic approach. please evaluate the body content if there is some. String sum = calc. This is a constant defined in the parent class. Name of tag 2.java
Step 2: Define the tag class in the tag library descriptor. “Hey container. It first calls all the setters and assigns the attribute values to the tag class properties. “Hey container. The doStartTag() method is usually empty and always returns EVAL_BODY_INCLUDE. It defines the following information: 1. Since our tag doesn’t have any body content. and computes the sum as shown below:
Calculator calc = new Calculator(). Entering in to this method means.setCount(count).

This file though contains xml content.//DTD JSP Tag Library 1. The elements like tlibverion.tld
Step 3: Importing the TLD file into the JSP page Remember we talked about JSP directives at the beginning and delegated the usage if third directive named taglib to later section. These are the required elements. Now is the time to use this.tld” prefix=”calc” %>
The above directive will tell the web container something like. the tag class.jspversion.0</tlibversion> <jspversion>1. and the list of attributes the tag will have.0" encoding="ISO-8859-1"?> <!DOCTYPE taglib PUBLIC "-//Sun Microsystems. “Hey container.Chapter 14
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See the following tag library descriptor for our custom tag shown below:. Save this file in the following directory as:
/myweb/WEB-INF/tlds/mytags.
<calc:sum count=”100”/>
Listing 14. This directive is used to import the TLD file into the JSP page as shown below:
<%@ taglib uri=”/WEB-INF/tlds/mytags. Inc.tld” extension.tld) The custom tag library descriptor
<?xml version="1.1//EN" "http://java.11b (mytags.dtd"> <taglib> <tlibversion>1.com/j2ee/dtds/web-jsptaglibrary_1_1. The above code is pretty obvious and conveys a clear picture.CalculatorTag</tagclass> <bodycontent>JSP</bodycontent> <attribute> <name>count</name> <required>true</required> <rtexprvalue>false</rtexprvalue> </attribute> </tag> </taglib>
The only important element in the above XML is the one highlighted in bold. The important one is the tag element which defined the name. must be saved to a file with a “. this JSP uses custom tags that are defined in mytags.sun.shortname and info are used to convery some basic information to the container.tld file which has all the information to process the
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.1</jspversion> <shortname>examples</shortname> <info>Simple Library</info> <tag> <name>sum</name> <tagclass>customtags.

Listing 14. Type the following URL in the browser
http://localhost:8080/myweb/jsps/CalculatorTag. There is nothing to explain here guys.
C:/>jakarta-tomcat-4.>classes>javac customtags\*. Start the server using the following command and wait for few seconds. Moreover.tld" prefix="calc" %> <calc:sum count="100" />
Steps to run the JSP 1. Save all files in the following directories
/myweb/jsps/CalculatorTag..11c (CalculatorTag.tld
2.java
3.. Look at the following code that covers both Step 3 and 4.Chapter 14
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tags. Compile the classes as shown below:
C:/>JavaTraining>.java /myweb/WEB-INF/classes/tlds/mytags.1.jsp
The above URL produces the following result:
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4.jsp) JSP using a custom tag
<%@ taglib uri="/WEB-INF/tlds/mytags. Simply use the above custom tag in the JSP page.>.jsp /myweb/WEB-INF/classes/customtags/CalculatorTag. all the tags defined by this TLD file will be prefixed with calc” as shown below:
<calc:sum count=”100”/>
Step 4: Using the custom tag in the JSP page.

The body content can either be html or JSP as shown below: HTML body content
<calc:sum count=”100”>Hello John</calc:sum>
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You can now update the JSP to use the custom tag again and again as shown below:
<%@ taglib uri="/WEB-INF/tlds/mytags.tld" prefix="calc" %> <calc:sum count="100" /> <calc:sum count="125" /> <calc:sum count="150" /> <calc:sum count="175" />
The above code produces the following result:
We can also specify the body content for a custom tag.

you are free to write your own custom tags just in case if the functionality you are expecting is not available with the built-in ones. ok. At this point.
Summary
JSP is a reverse technology to Servlets. simple concepts are easy to forget.
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.CalculatorTag</tagclass> <bodycontent>JSP</bodycontent> </tag>
Recognizing the important of custom tags in JSP pages. Most of the real world applications uses these technologies and leverage the built-in custom tags to use in JSP pages. companies like SUN and Apache Software came up with technologies with rich set of ready made custom tags. Java code is embedded in html code. Always remember that. we finished learning all the important concepts in JSP technology. These are our next chapters. I am sure you really enjoyed JSP programming and learnt to use several cool features it offered. include and taglib defines the elements that effects the overall structure of the page. So. follow the above procedure. These companies identified the most common custom tags that any JSP page requires and provided the implementations for the tags by themselves. Scriplets are used for generating the dynamic content. The web container translates JSP into a Servlet JSP uses special placeholders for Java code. In such cases. The two most popular technologies that had rich set of custom tags are JSTL which stands for Java Standard Tag Libraries and Struts created by Apache. read this chapter twice before you go to the next one. to simplify our life from building custom tags from scratch. In JSP technology. You can now use combination of Servlets and JSP to build any complex web application. The JSP directives namely page.Chapter 14
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Summary
JSP body content
<calc:sum count=”100”> Hello <%= name %> </calc:sum>
All you need to do is in the TLD file. JSP declarations are used for declaring global methods and variables. JSP technology is built over servlet technology. However. specify the body content for the tag as html or JSP as shown below:
<tag> <name>sum</name> <tagclass>customtags.

JSP uses Custom tags to build html markup to be used any where in the page. out and session. Which of the following standard action is used for defining a Java bean? a) <jsp:getBean> b) <jsp:useBean> 6. JSTL and Struts are popular technologies that include several built-in custom tags.
Time to play 50-50
1. A JSP page can use the implicit objects namely request. These are the most widely used objects. Which of the following directive is used for importing the custom tags descriptor? a) include b) taglib 3. Which of the following placeholder is used to define a scriptlet a) <% %> b) <%= %> 5. Which of the following is used to retrieve the bean property?
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. Which of the following page attribute allows session management? a) session = “true” b) scope=”session” 4. JavaBeans are used to isolate business logic from presentation logic. JSP uses standard actions to work with JavaBeans. JSP uses standard actions to store and retrieve the data in and from the JavaBeans.Chapter 14
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Time to play 50-50
JSP expressions are used for echoing the dynamic content. Which of the following directive is used for importing the resources? a) include b) page 2.

out. Which of the following file is used to define custom tags? a) TLD file b) Text file 10. Which of the following tag technology is created by Sun? a) Struts b) JSTL
Interview Questions
Question: What are the implicit objects in a JSP page? Answer: request. Which of the following directive is used to include the response of another resource? a) getResource b) include 9. Question: What is a custom tag? Answer: Custom tag is a user defined tag that conveys a special meaning to the container. session. Which of the following is an implicit object? a) request b) HttpRequest 8. response. Question: List the steps involved in writing a custom tag
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a) <jsp:property> b) <jsp:getProperty> 7. The content can be either HTML or JSP. context Question: What the difference between include directive and standard action <jsp:include> Answer: Directive inclusion provides static inclusion of content and <jsp:include> action provides dynamic inclusion of content.

but uses “.tld” extension. Question: Where do you configure custom tags? Answer: Tag library descriptor.Chapter 14
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Interview Questions
Answer: Refer to Custom tags section. Question: What is a scriptlet? Answer: A scriptlet is a piece of Java code embedded in JSP page to generate the dynamic content. which is an xml file.
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it would be really nice if we have ready made useful custom tags that we can use in our JSP pages without having to develop them. It is a set of cool ready made custom tags. Though we know how to build custom tags on our own. 2. displaying and manipulating variables in specified scope etc. Let me tell you one thing here. Following are the important ones. conditions etc. So. Core Tags SQL Tags XML Tags Format Tags
Out of the above. Each category has like 5-6 tags and are very simple to use and hardly takes few minutes to master them. What we will do is. This is what JSTL is all about.
Core Tags
These tags are used for general purpose page actions like conditions. core tags are the widely used ones followed by SQL tags. iterations. Since we are in competitive world in which time is very precious. Simple to use Enhances JSP page readability Highly powerful and Faster JSP development
This chapter gives you an idea about how to use different variety of JSTL tags in JSP pages. 2. JSTL tags are broadly classified into four groups as shown below: 1. 3.
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. the only concern is that we have to develop them from scratch. Using these tags has become a common practice in most of the real world applications due to the following reasons: 1. Let's see what these are. In the last chapter we saw how to develop custom tags for JSP pages. 4. 3. 4.Chapter 15
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Introduction
This chapter introduces you to JSTL which stands for Java Standard Tag Library. JSTL provides us with custom tags that can be used for most of the common actions in JSP pages such as iteration. All the core tags will be prefixed with "c". learn all the tags in each category and then do an example using the same. without wasting any further time let’s see what these are.

then it displays NO DATA in the browser. Usually we store the data in request or session scope. I know you are seriously wondering about what "${ }" is for. Example 3:
<c:out value="${data}" default="NO DATA" />
In the above tag. if the value of data variable is null. we can use this tag as shown below:
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<c:out value="" default=""/> This tag will display what ever you pass into value attribute. The symbol $ should be read as "Value of variable or bean property" and the variable/property name should be enclosed within curly braces.The default attribute specifies the default value to be displayed if the value attribute evaluates to null. Example 1:
<c:out value="Hello World" />
prints "Hello World" in the browser. <c:set var="" value="" target="" property="" scope="request/session" /> This tag is used to store data in the specified scope as name-value pair. %> <c:out value="${str}" />
The above snippet prints the value in the scripting variable str which is Cool JSTL Use this tag where ever you want to display dynamic content. str += " JSTL". Example 4: To store a value something like Steve in session with the key user. We need to use this to display the value of a page variable or a bean property.
Example 2:
<% String str="Cool".

<c:if test="" /> This tag is used to test a condition. This is the beauty of custom tags. We can then print the object using the c:out tag as shown above. }
As you can see.out. The above tag should be read as. System. Example 6:
<c:if test="${accountExists}" > Total Balance in the account is $<c:out value="${totalFunds}" /> </c:if>
The above tag is equivalent to the following Java code:
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. If the condition is evaluated to true then the statements inside the tag will be evaluated. <c:forEach var="" items="" begin="" end="" ></c:forEach> This is the most widely used tag for iterating through a collection of objects.Chapter 15
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<c:set var="user" value="John" scope="session" />
Example 5: To store some value in a Java Bean property we use it as
<c:set target="customer" property="ssn" value="12345" scope="session" />
The above tag stores the value 12345 in the ssn property of customer bean in session.i++) { Object str = list.get(i). the custom tag representation is more readable than the equivalent Java code representation. display item. Look at the following implementation for this tag. for each object in list taken into item.
<c:forEach var="item" items="${list}" > <c:out value="${item}"/> </c:forEach>
The items attribute take the collection to iterate and assigns every object in the collection to item variable.size(). The above tag is equivalent to the following Java code:
for(int i=0.i<list.println(str).

This tag is normally used like a typical if-else block.out. See the code in listing 15. Example 7:
<c:choose> <c:if test="${accountExists}" > Total Balance in the account is $<c:out value="${totalFunds}" /> </c:if> <c:otherwise > Account not found.1. if the account doesn't exist then the content enclosed in the c:otherwise tag will be evaluated. }
<c:choose ></c:choose > This tag is used in conjunction with <c:if> tag and <c:when> tag.
http://localhost:8080/myweb/PagingServlet?page=2
We can then use the above URL in a hyperlink element as shown below:
<a href='<c:out value="${nextpage}"/>'> Click Here to go to next Page </a>
This completes all the important core tags.println ("Total Balance in the account is $ " + totalFunds").
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. As I promised before. let's do an example using all the above core tags. </c:otherwise > </c:choose>
In the above snippet.Chapter 15
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if ( accountExists == true){ System. <c:url var="" value""> <c:param name="" value=""> The above two tags are used to create a URL and append query parameters as shown below:
<c:url var="nextpage" value="http://localhost:8080/myweb/PagingServlet"> <c:param name="page" value="2"/> <c:url>
The above code snippet will produce the following URL and stores it in nextpage variable.

All the SQL tags are usually prefixed with the word sql. These tags allow the page developers to execute SQL queries from the page itself with minimum effort.mysql. The SQL tags as the name suggests are used for database operations. Example 1:
<sql:setDataSource var=”myDatSrc” url=”jdbc:mysql://localhost:3306/MyDB” driver = “com. Good.jsp The above URL produces the following output
SQL Tags
I am sure you already guessed what these tags have to offer. We can now use this variable to execute the SQL queries. Let’s see the important ones here. <sql:setDataSource> Following is the syntax for this tag. <sql:setDataSource var=”” url=”” driver=”” username=”” password=”” /> This tag is used to establish a connection with the database with the given information.Driver”/>
The above tag gets a connection to the MyDB database and stores it in myDatSrc variable.
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http://localhost:8080/myweb/jsps/CoreTagsDemo. The connection object will be returned in a variable specified by the var attribute.

This tag will then return the results object that we can iterate and display the results. We need to use the <sql:param> tag with the value attribute as shown above.
<sql:query var=”results” sql=”Select * from Customers” dataSource=”${myDatSrc}”/>
(or)
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. The query for this tag can be specified as the body content or through the sql attribute. we use the second convention as shown below
<sql:update dataSource=”${myDatSrc}”> INSERT INTO CUSTOMERS VALUES (?. <sql:query> This tag is used to execute the SELECT queries. the select query can be supplied as an attribute or body content.?) <sql:param value=”${firstName}”/> <sql:param value=”${lastName}”/> </sql:update>
The important thing with this convention is the way we set the query parameters. The syntax for this is shown below <sql:query var=”” sql=”” dataSource=”” /> Like the update tag. In our query we plugged in the firstName and lastName as two parameters. Following snippets show both the cases.’Bond’)” dataSource=”${myDatSrc}”/>
If the parameters are required to be passed dynamically which is usually the case.Chapter 15
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<sql:update> This tag is used for executing all the SQL queries except the SELECT query. The syntax for this tag is shown below: <sql:update sql=”” dataSource=””/> or <sql:update dataSource=””>SQL </sql:update> Use the first convention if the SQL is static and doesn’t require setting any parameters dynamically as shown below:
<sql:update sql=”INSERT INTO CUSTOMERS VALUES(‘James’.

jsp) JSP using a JSTL sql tags
<%@ taglib prefix="sql" uri="/WEB-INF/tld/sql.tld"%> <%! int count = 1. String ln = "Bond" + count.lastName}”> <c:out value=”${record.tld"%> <%@ taglib prefix="c" uri="/WEB-INF/tld/c. FirstName LastName SSN James Bond 99999 Rob Smith 12344 The results variable should be iterated as shown below to display the data. See the code in listing 15.rows}”> <c:out value=”${record.firstName}”> <c:out value=”${record. Its usage is pretty simple as shown below:
<sql:transaction> <sql:update sql=”” dataSource=”${myDatSrc}”/> <sql:update sql=”” dataSource=”${myDatSrc}”/> </sql:transaction>
All we need to do is enclose all the queries that are to be part of the transaction in the body of the tag.ssn}”> <c:forEach>
<sql:transaction> This tag is used to specify multiple queries in a single transaction. Listing 15.
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<c:forEach var=”record” items=”${results.Chapter 15
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<sql:query var=”results” dataSource=”${myDatSrc}”> Select * from Customers where ssn=? <sql:param value=”${ssn}” /> </sql:query>
Let us assume that the above SQL returned the results as shown in the following table. String fn = "James " + count. Let’s do an example. This completes all the important SQL Tags.2 (SQLTagsDemo. %> <% count++.

Most the XML tags except the parse tag are similar to core tags with just a small difference in the sense these tags use the attribute select instead of value or items attributes. These tags basically use the XPATH structure to retrieve the data from xml. then we can use this tag as shown below:
<x:parse xml=”${xmlData}” var=”data”/>
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. For instance. Usually XML tags are prefixed with “x”. Following are important XML tags. or the variable name that contains the xml content. The syntax for this tag is shown below: <x:parse xml=”xml source” var=”variable name” /> The XML source can be the URI to an xml file on the server. look at the following usage.
<x:parse xml=”/xmls/Messages.xml file in the directory named xmls. If the xml content is stored in a page variable. XML tags are used for parsing the XML content and then displaying the nodal information.xml” var=”data” />
The above tag parses the Messages.Chapter 15
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XML Tags
XML Tags are used for processing XML documents. <x:parse> This tag is used to parse the given XML document.

Using the reference variable.Chapter 15
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Once the XML parsed. we can retrieve the values in various nodes. Consider the following XML. it stores all the nodal structure with the reference variable specified by the var attribute (data).
<response> <messages> <message> <text>This is message 1</text> <id>1234</id> <status>Processed</status> </message> <message> <text>This is message 2</text> <id>12345</id> <status>Processed</status> </message> <message> <text>This is message 3</text> <id>12346</id> <status>Failed</status> </message> </messages> </response>
When the above XML is parsed and referenced with the variable data. <x:forEach> This tag is used to iterate over the XML elements using the XPATH representation. we can use the forEach tag to display the contents of repititive elements (message) as shown below:
<x:forEach var=”element” select=”$data/response/messages”> <td> <x:out select=”$element/text” /> </td> <td> <x:out select=”$element/id” /> </td> <td> <x:out select=”$element/status” /> </td> <x:forEach>
<x:if> This tag is used to evaluate the body of the tag based on the XPATH expression as shown below:
<x:forEach var=”element” select=”$data/response/messages”> <x:if select=”$element/status == Processed”> <td> <x:out select=”$element/text” /> </td> </x:if> <x:forEach>
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.

jsp
This completes all the JSTL tags that we need to know for JSP page development. SQL Tags. XML tags are used for processing xml content. SQL tags are used for working with databases by executing SQL queries. since these are the widely used ones. Just read the core tags twice. Save the JSP code as “myweb/jsps/XMLTagsDemo. These tats use XPATH syntax for retrieving the XML content. Start the server and type the following URL
http://localhost:8080/myweb/jsps/XMLTagsDemo.Chapter 15
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3. These tags provide the basic actions in JSP such as iterations. JSTL tags are extensively used in real world applications and using them makes the page look much cleaner and simple. I am sure you found the JSTL tags very helpful and easy to use them in JSP pages.
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.
Interview Questions
Question: What the different types of JSTL tags? Answer: Core Tags. XML Tags and Format Tags.
Summary
JSTL is a standard library of Custom tags that can be readily used in JSP pages without building from scratch. JSTL tags are classified as Core tags. The most frequently used ones are the core tags. conditions etc. XML tags and Format Tags.jsp” 4. Let’s summarize this chapter. SQL Tags.

apache.cgi 2. This is an open source framework developed by Apache Software foundation. /myweb/WEB-INF/classes/properties
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.jar) to the following “lib” directory in your web project. http://struts. Using Struts Validator Using Struts Tiles Internationalization using Struts
Environment Setup
1.
/myweb/jsps/struts. By the end of this chapter. Copy all the Jar files in the Struts installation directory (jakarta-struts\lib\*. /myweb/WEB-INF/classes/formbeans /myweb/WEB-INF/classes/actions.org/download.
/myweb/WEB-INF/lib
3.
Chapter Goals
Understand MVC Design Pattern Using Struts components.Chapter 16
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Chapter Goals
Chapter 16
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This chapter introduces to the most popular and widely used web technology called Struts. Download the Struts framework at the following URL. Create the following directories to store Struts components. you’ll understand the important features of Struts and how web applications are built using it.

a framework doesn't introduce you to new type of programs. Two important to remember here: Point number one: Struts framework comes with several built in classes and interfaces. there is always a good design behind a successful framework. You’ll agree with me by the end of this chapter. store it in some directory. because it makes life easy.a MVC. However. but using its new approach or process. You'll like it. The process that struts follow is very simple. A framework typically doesn't reinvent the wheel. What. we don't need to know about all of them. Let’s see what MVC design is before learning something about Struts
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. By following its style (process) we can build some cool web applications.Chapter 16
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Introduction
Struts is an open source framework developed by Apache Software foundation for building dynamic web applications. and how they help us and all that good stuff later. one more framework to build web applications? Thankfully Yes. Point number two: Like there is woman behind a successful man. The good thing is that struts framework is built using same technologies but uses a different approach. but it uses a good design to build a better and reliable wheel. The design behind Struts is the popular design pattern. They are very simple. But there are few classes that we need to use to build web applications. “The Model View Controller” a. In earlier chapters we looked at Servlet and JSP technologies for building web applications. Therefore. For instance. Struts framework also has its own style (process) based on a design called MVC and we follow this process to build robust dynamic web applications in Java. The reason I used the word ‘thankfully’ is because this is such a good framework and makes life much easier with web application development. but in a different style. Struts is one such framework for building web applications in Java. then define the class in an XML file and store it so and so directory and so on. What I mean to say is. we will write a Java program that is derived from so and so class. we will still continue to write the same old JSP pages. but it will use the same old technologies. Generally the term framework means some kind of ready made infrastructure based on a very good design using which one can build web applications at faster pace and that are flexible and easy to maintain. We'll see what these classes are. trust me.k. simple Java programs etc.

there will only be one controller servlet in this design and this why we say that the controller as a centralized component.
Browser JSP Java Beans ( Models) Model View Design ( MVC 1)
Browser Servlet Java Beans ( Models)
Model View Controller Design ( MVC 2) Fig 16. and as a view it also displays the information to the user. Moreover. This design is popularly known as MVC2.Chapter 16
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Model View Controller Design
Model View Controller Design
When we learned Servlets and JSP technology. In this design. all we did is cleanly separated controller duties from the view component into a servlet. processes the request using the Java Beans (Models) and sends the response back to the browser. it will select the appropriate JSP page to display the information in the browser.1 MVC1 and MVC2 designs
The top figure shows the Model 1 design in which a JSP (View) takes the requests from the browser. It can only send requests to a servlet. Once all the data is processed and models are prepared to encapsulate all the information. In this design JSP acts as both the controller component as well as the view component. which will use its own expertise or helper classes to process the request. This design is referred to as MVC1 design. it intercepts and processes the requests by dispatching to helper classes. As a controller. unknowingly we used two types of design processes namely Model 1 design and Model 2 design. Now look at the second figure which shows the Model 2 design.
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. I am sure you already guessed how the design works. look at the following two figures. To better understand these two designs. The browser can no longer send requests to JSP pages.

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. a POJO . It comes with Tiles framework that we can leverage to build composite pages with flexible page layouts. 8. It has excellent support and ready made infrastructure for data validation. "n" number of Views that queries or updates different models.Plain Old Java Object) View: A component that sends queries or requests to update the model data. To summarize. 6. 2. JSP acts as both controller and view. 3. This is the design that Struts framework use to build dynamic web applications in Java. Excellent support for error handling. and in MVC2 design. Allows building web applications that are flexible and easy to maintain. let’s get into serious Struts business. Its main duties are: 1) Processing the requests and preparing the models 2) Select the View to display the models. Controller: A centralized component that reads the requests from the browser or a view. Model: An entity that stores the data (A Java Bean.
Why Struts is important to us?
Here are the few reasons why we are interested in Struts framework. 1. This is an interview question. It manages all the interactions within the application by coordinating with other components. Note this point. 5. Uses XML for configuring the applications. In MVC2 design. Follows a systematic process to build applications. It has inherent support for internationalization making web applications run in different locales. and "1 and only 1" Controller. 4. following are the key components of MVC2 design. Comes with several cool and ready made custom tags to be used in JSP pages. 7. there will be "n" number of models to store different variety of data. a Servlet acts as a centralized controller and JSP acts as a View component. or to read the model data. Now that we know what this design is and how it works. in MVC1 design. It acts as a single point of entry and exit to/from the application.Chapter 16
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Why Struts is important to us?
The only difference between the two designs is that.

Follow the bolded arrows starting from 1. this is how the story builds up: 1. The above features are one of the reasons why Struts has tasted instant success and became a de facto standard for all the real world web applications built in Java. If you read carefully. I am sure you can’t wait to learn Struts.
How Struts work?
Look at the following figure. and read the text in the boxes and along the arrows. Browser sends request to Controller (ActionServlet class) 2. Controller loads the information related with request from the XML file and
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.2 Struts flow of control
I hope the above picture is clear and give you some idea about how Struts work.Chapter 16
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How Struts work?
With all the above good things that Struts framework has to offer.
6
Give all this information to
3 FB 1 populates form bean RP 1 creates request processor 1 sends request to Browser Controller RP 3 sends response to 7 XML Properties 2 RP 2 FB 3 Ac 3 Ac 2 sends FB1 to 4 FB 2 Ac 1 5
Read the form bean data Call the backend and get the response Populate the models with information Select the view to display results
JSP
JSP
RP n
Ac n
Views
Models
Fig 16.

it does two things.
Struts API
The Struts API is a set of classes and interfaces that we normally use to build application components.Chapter 16
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Struts API
creates a RequestProcessor by passing the information to it (RP 1) 3. Now that we know how Struts work. 5. They are internally used by Struts framework. Besides these. In the above figure. The struts framework will take care of the rest. get the response and 3) Populate the models (Java Beans) and store them in request scope or session scope. 4) Select the JSP that will use the models. 1) Read the data in the request and populate the Form Bean (FB1) 2) Send the form bean as a parameter to Action class (Ac 1) 4. Once we finished developing all these components. does the following 1) Read the data from the Form bean 2) Process the data. Table 16.xml. We don’t have to create them or write them. Action class after receiving the form bean.1 Struts API Class/Interface ActionForm Description This class is used to create form beans
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. Based on the information the request processor (RP1) got from controller. we also have to write the JSP pages. The form beans and action classes are the ones that we need to create as part of development. Writing these classes hardly takes couple of minutes. Following table lists the most important and widely used classes. some components are ready made and some of them are not. let’s see some of the components that we use to build web applications. we define them in a configuration file named struts-config. The action class will supply all the information to the Controller which will then use the selected JSP to send the response to the browser. The ready made components are controller and request processors.

the controller servlet intercepts all the requests sent by the browser to a struts based web application. is the way the HTTP requests are send by the browser.do The struts controller component will intercept the above request and interpret it in the following manner.do extension. Therefore it gets /welcome as the name of the request. See if there is information defined in struts-config.do” extension as shown below: http://localhost:8080/myweb/welcome. The controller usually expects all the requests from the browser to end with a “. property file: This file defines all the struts related and application related message properties which is used for displaying messages to the user.xml with the name /welcome. 2. can you tell me the name of the file in which this servlet will be configured? Good. There are two important files we need to know for Struts development. I will explain the usage of the above classes when we start writing example programs. If the information is found. The controller component is a servlet named ActionServlet. These are: struts-config. It’s web.Chapter 16
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Action ActionMapping ActionError ActionErrors DynaFormBean
This class processes http requests Class used for selecting the view components Class used for defining an error Class that represents a collection of errors Class that is used for working with forms and validation
The best way to learn the API is by using the API. Get the name of the request.xml file we used to configure servlets. it will read the information and initiates processing the request. The name if the request is the part of the URL after the context name myweb and before .xml: This file is where we configure all the Struts related components.
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. Since this is a servlet. As we learned couple pages ago. One of the conventions that stand on top of all. 1.
Struts Requests
Struts framework follows some conventions.

the configuration file that struts uses is struts-config. These are the only two cases you'll normally see in any struts based application.Message Resources --> <message-resources parameter="props. In this case.xml file. Scenario 1: This is the scenario without action class.xml file
<struts-config> <!-. the controller will delegate the request to a RequestProcessor object which will use the formbean (if any) and action class to process the request. we commonly see the following two scenarios for processing the requests.Global Exceptions --> <global-exceptions> </global-exceptions> <!-. Let’s see what configuration information this file defines. Following is the typical structure of the struts-config.Chapter 16
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The struts configuration file
Typical Struts Scenarios
In Struts based applications. In this case.xml just like servlets use web. The action class finally returns the response back to the controller which will then send it to the browser.
The struts configuration file
As said before.Data Sources --> <data-sources> </data-sources> <!-.Action Mappings --> <action-mappings> </action-mappings> <!-.Global Forwards --> <global-forwards> </global-forwards> <!-. All the examples we do in this chapter also fall in one of the above cases. Scenario 2: This is the scenario with action class.Form Beans --> <form-beans> </form-beans> <!-.MessageResources"/>
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. the controller will use its own expertise with the information defined in the configuration file to process the request.

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<!-- Message Resources --> <plug-in> </plug-in> </struts-config>

<data-sources> This tag is used for defining the data sources to use within the application. This tag is rarely used. <form-beans> This element is used to define all the form beans within the application. This element uses the child element <form-bean> for defining individual beans as shown below:
<form-beans> <form-bean name=”customer” type=”formbeans.CustomerBean” /> </form-beans>

The above defines a single formbean with the name customer that belongs to a class CustomerBean in the formbeans package. <global-exceptions> This element is used to define error pages for various exceptions thrown by the application components. Its usage is as shown below:
<global-exceptions> <exception type=”java.io.IOException” path=”Error.jsp”/> </global-exceptions>

If any of the application components throws IOException, then the controller will forward the request to the Error.jsp page. <global-forwards> This element is used to declare the global forward pages that can used by all the action classes. Its usage is shown below:
<global-forwards> <forward name=”success” path=”/jsps/Main.jsp”/> <forward name=”failure” path=”/jsps/Error.jsp”/> </global-forwards>

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With the above forwards, if the action class selects the success forward, then the controller will forward the request to Main.jsp. <action-mappings> This is the most important element in the configuration file. It defines all the action classes and related information like exceptions, forwards etc. We’ll see the details of this element when we do examples. <message-resources> This element is used for internationalization support. It defines the list of property files and resource bundles to be used by the framework to display messages on the browser based on locale. <plug-in> This element is used to plug in external frameworks into struts framework. This is one of the key features of struts, the ability to integrate with other frameworks. With this basic understanding of what a struts configuration file contains, let’ begin our journey into Struts. Two simple tricks 1. If the request sent from the browser doesn’t contain any data, then we don’t need to write formbean and action class. We just have to define an action mapping in the configuration file with information sufficient for the controller to take care of the request. 2. If the request does include data, then we must write an action class. If the data comes by submitting a HTML form, then we also need to write a formbean class. Otherwise, no form bean is required. Let’s write a simple program to get started. This example simply takes a “.do” request from a browser and forwards it to a welcome page. Since no data will sent in the request, we use trick 1. See the code in listing 16.1. Listing 16.1a (Welcome.jsp) Simple JSP page
<HTML> <HEAD>

Steps to run the program 1. Save the JSP page in the following directory as
/myweb/jsps/struts/Welcome.jsp

2. Save the configuration as
/myweb/WEB-INF/struts-config.xml

3. Start the server.

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4. Type the following URL in the browser
http://localhost:8080/myweb/welcome.do

When the above URL is sent to the controller servlet, it checks for action mapping whose path is /welcome. Since we defined the following as the action mapping in the strutsconfig.xml file,
<action path="/welcome" forward="/jsps/struts/Welcome.jsp"/>

the controller will forward all the /welcome requests to Welcome.jsp page. The above URL will therefore produce the following result:

The above example is very simple and I am sure you got it in few seconds. In the next example what we will do is, send a request parameter called page with two possible values. Based on its value, we will have the controller forward it to appropriate page. In this example, we will create two JSP pages namely Sales.jsp and Inventory.jsp. When the browser sends the following maintenance request, the controller should display Sales.jsp content:
http://localhost:8080/myweb/maintenance.do?page=sales

Similarly, for the following URL, it must forward it to Inventory.jsp
http://localhost:8080/myweb/maintenance.do?page=inventory

4. Start the server. 5. Type the following URL in the browser
http://localhost:8080/myweb/maintenance.do?page=sales

The above URL produces the following result

Now change the URL to send inventory as the page parameter as shown below:
http://localhost:8080/myweb/maintenance.do?page=inventory

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Since the example worked the way it should, let’s understand how to build action class.

Action Class
Action class is a core component in the struts framework that we need to write to process the request. To create an action class, 1. Write a class that extends org.apache.struts.Action. 2. Overwrite the following method in the Action class.
public ActionForward execute(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception

The above execute() method takes the following four arguments: 1. ActionMapping - Used by the action class to select the view (JSP page) 2. ActionForm - This argument represents the formbean if there is one. Since our example doesn’t have any formbean class, it will be null and we don’t use this for now. 3. HttpServletRequest – This parameter is used to read request parameters and get a handle to session object. 4. HttpServletResponse – This is a response object, but we never use it. So, we will ignore this argument completely. The execute() method returns an object of ActionForward class. This object is used to return the selected view information back to the controller. Following is what we normally do inside the execute method:

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Read the request data
If a formbean is not defined by the application, we use the following methods to read the data based on where the data is present.
request.getParameter() (or) request.getSession().getAttribute()

If there is form bean defined, say CustomerBean, then we need to do the following: a) Get the form bean using the form argument as shown below:
public ActionForward execute ( ActionMapping mapping, ActionForm form,…){ CustomerBean bean = (CustomerBen) form; }

b) Read the data from the above bean using its get methods.

Process the Data
This is the step where we process all the data and generate dynamic content by calling whichever backend systems we want to. If there is no dynamic content, we simply select the next view (page) to go to. However, if there is dynamic content (which is normally the case) we store it in session and then select the next view. Make sense?

Return the View
Once the above step chooses the view, the execute method returns it back to the controller using the mapping argument as shown below:
mapping.findForward(nextPage);

In our example, we just followed the above process. Since we don’t have any form bean, we read the data as,
String page = request.getParameter(“page”);

We then assigned the value of the above variable to nextPage. This variable can have either sales or inventory based on what value is sent in the request, right? We then returned the view using the following statement.
mapping,findForward(nextPage);

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The above statement will return either sales or inventory back to the controller to select the view. Can you tell me how the controller associates the above mapping to the actual JSP pages? Look at the following action mapping we added in the configuration file:
<action name="/maintenance" type="actions.MaintenanceAction"> <forward name="sales" path="/jsps/struts/Sales.jsp" /> <forward name="inventory" path="/jsps/struts/Inventory.jsp" /> </action>

I am sure you figured it now. If the MaintenanceAction selects either sales or inventory view, the controller tries to locate the JSP in the forwards defined in the action mapping as shown above. What if the above forwards are not defined? You’ll get a “Page Not Found” error. In this example, our action class can only return sales or inventory as the forward. In the next example, instead of sending a request from the browser, let’s have a html form post the login information to a login action. This example will use a form bean to store the form data. Let’s first write the code and then look at the details. See the code in listing 16.3. Listing 16.3a (Login.jsp) Simple JSP
<html> <BODY> <P>Please Login into the application.<BR> <BR> </P> <form action="/myweb/login.do"> <TABLE border="1"> <TBODY> <TR> <TD>UserName</TD> <TD> <INPUT type="text" name="username" size="20"> </TD> </TR> <TR> <TD>Password</TD> <TD><INPUT type="password" name="password" size="20"></TD> </TR> <TR> <TD colspan="2" align="center"><INPUT type="submit" value="Submit"></TD> </TR> </TBODY> </TABLE> </form> </BODY> </html>

4. Start the server. 5. Type the following URL in the browser
http://localhost:8080/myweb/jsps/struts/Login.jsp

The above URL produces the following result

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Now that you successfully executed the program, let’s see how it works. The login page posts the login information to /login action as shown below:
<form action="/myweb/login.do">

The controller will then retrieve all the information form struts-config.xml for this action. Following is what it finds:
<action path="/login" name="loginForm" type="actions.LoginAction"> <forward name="success" path="/jsps/struts/Main.jsp" /> </action>

The above action mapping is almost same as the previous one but includes a name attribute. This attribute will tell the controller that the LoginAction class is using a formbean named loginForm to collect the data. The controller goes to the form beans mappings in the configuration file, and looks for the bean definition with the same name and finds the following:
<form-bean name="loginForm" type="formBeans.LoginForm"></form-bean>

Now look at the form bean class LoginForm. Two simples rules to write this class: 1. This class should inherit from ActionForm class 2. Define the form properties (names must match the html field names) with getters and setters. This is exactly what we did in the LoginForm class. When the login page is submitted, the controller will populate the formbean properties with the form data and passes the formbean as a parameter to LoginAction class. The action class will first read the formbean as shown below:
LoginForm loginForm = (LoginForm) form;

Once the bean is retrieved, it will invoke the getter methods to read the data.
String username = loginForm.getUsername(); String password = loginForm.getPassword();

At this point, you can do whatever you want with the data. In our case, we want to get the actual name of the user from the username to display it in the next page. For this, we called the getName() method passing the username. This method should ideally connect to database and get the name, but we just mocked up couple of names using HashMap object. If no name is found, it returns Guest. After the name is returned back,

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we just can’t leave it alone. Instead, we must store it in the session so that the next page can read it from the session and display it. This is exactly what we did with the following statements.
request.getSession(true).setAttribute("Name",name); nextPage = "success";

Now if you look at the action mapping for this action class, success will be forwarded to Main.jsp as shown below:
<forward name="success" path="/jsps/struts/Main.jsp" />

Finally, the main page will read the session attribute and displays the greeting. If the above explanation is confusing, here is the simple process that outlines the above: 1. HTML form is submitted to action class 2. Action class uses the form bean to collect data. So, the controller populates the bean and sends it to action class 3. Action class will read the form bean data, process it and selects the next page I am sure the above process is pretty straightforward. Important Trick: Whenever there is a html form, write a form bean with the form properties and use the formbean within the action class. This is also the reason why it got the name formbean. Now that you understood the basics of Struts, it time to concentrate on JSP page development using struts style. Once we learned to build JSP pages using struts style, we will again get back to normal struts business. Ok.

JSP development using Struts Tags
As I said before, Struts comes will several built in custom tags to assist in view (JSP) development. One of the main goals of Struts is to completely change the style of JSP development by using custom tags for everything including the standard HTML elements like textfields, hyperlinks, textareas etc. Let’s now see how we can use Struts custom tags to build JSP pages. All the custom tags in struts are categorized into four types as shown below: 1. html tags (Used to build HTML elements)

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we import them in the JSP pages as shown below:
<%@ <%@ <%@ <%@ taglib taglib taglib taglib uri="/tags/struts-html" prefix="html" %> uri="/tags/struts-tiles" prefix="tiles" %> uri="/tags/struts-logic" prefix="logic" %> uri="/tags/struts-bean" prefix="bean" %>
Based on the above taglib directives. Following is the URL to the tags documentation: http://struts. 2. tiles tags (Used for page layout) The struts installation will include the TLD files for all the above tags. logic tags (Used to do some logical operations like looping) 4.tld struts-bean. I know it’s tough to memorize all of them. <tiles:insert…… and so on..tld struts-logic. 3.tld struts-tiles.9/userGuide/
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. struts-html. 4.org/1. There is good documentation available in Apache Struts website with complete information about each and every tag and how to use it. all the bean tags will be prefixed with bean etc.Chapter 16
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2. <bean:define ….2. The usage of these tags is pretty simple and again hardly takes few minutes to master them.. so we will only look at the most important ones here. bean tags (Used to process the models or Java beans) 3. Ok.
Let me tell you one thing here. you can easily guess that all the html tags will be prefixed with html.tld
To use the custom tags defined in the above TLD files. as shown below:
<html:link….apache. <logic:iterate…. Following are the four TLD files for the above four categories of tags: 1. There are several custom tags in each category unlike in JSTL where we have few tags.

All the bean tags are normally prefixed with the word bean.accountType and so on. Based one the above bean structure. The syntax is shown below:
<bean:define id="" value="" /> <bean:define id="" name="" property="" scope=””/>
Example 1: The following defines a scripting variable named userSSN and assigns the value 12345. given an account bean. address. The AccountDetail bean in turn has two properties. the account type is retrieved with the property key accountDetail.
<bean:define id="type" name="account" property="accountDetail. You can then use userSSN variable any where in the JSP page. array of creditcards and a bankName. let’s look at the important tags. let’s consider a bean structure as shown below:
Account accountDetail: AccountDetail accountType accountBalance address: Address addressLine1 city state country creditCards[]: CreditCard name number bankName: String
The Account bean has three properties namely accountDetail. To better understand how the bean tags work. Now.accountType" scope=”session”/>
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. Creditcard has two properties and Address bean has four properties. The Account bean is the top level bean that represents all the information. <bean:define> This tag is used to define a scripting variable in a JSP page.
<bean:define id="userSSN" value="12345" />
Example 2: The following defines a scripting variable named type and assigns the value of accountType property of account bean stored is in the session.Chapter 16
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Bean Tags
These tags as the name suggests are used for reading and populating the bean properties.

<bean:message key="error. Its syntax is shown below:
<bean:write name="" property="" />
Example 3: The following usage prints the value in bankName property of account bean stored in session.name" scope=”session” /> <bean:write name="account" property="creditcards[0]. 1.
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.
<bean:write name="account" property="bankName" scope=”session” />
Example 4: The following usage prints the value in name and number properties of first creditcard in account bean stored in session.username key from the property file. Write an action class to populate the Account bean and forward it to JSP page named BeanTagsDemo.
<bean:write name="account" property="creditcards[0]. Use the bean tags in the JSP and display the Account bean properties. let’s write an example using the above tags.jsp 3. What we will do is.username" />
Now.number" scope=”session” />
<bean:message> This tag reads the property value from the properties file based on the specified key.Chapter 16
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<bean:write> This tag echoes the value stored in a bean property on the page. Create the above Account bean structure 2. Its systax is shown below:
<bean:message key="" />
Example 5: The following displays the property value of error.

setAccountType("Checkings"). let’s send a parameter named page whose value is used to select the view. account. address. Account account = new Account().setState("NE").setBankName("Chase").account). detail. Finally.setAttribute("account".setAddress(address).findForward(nextPage)). account.setAccountBalance("2000. save the specified key(s) return (mapping.tld" prefix="bean"%> <html> <BODY> <P> <h2> Accessing the Account bean properties from the session </h2></BR> <bean:define id="ssn" value="324324" /> <bean:write name="account" property="accountDetail. detail. account.setCreditCards(cards). account. we first created and populated the child beans.setAddressLine1("1111 S St"). we stored the Account bean in the session with the name account as shown below:
request.Chapter 16
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AccountDetail detail = new AccountDetail().accountBalance" scope="session"/>
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. } }
As you can see from the above code. } catch (Exception e) { } // If a message is required.getSession(true). Address address = new Address().setAttribute("account".30"). address. account).jsp) JSP page using bean tags
<%@ taglib uri="/WEB-INF/struts-bean.4f (BeanTagsDemo. and then added the child beans to the Account bean using the set methods.setAccountDetail(detail). Listing 16. address.getSession(true).
In the request from the browser. Following is the JSP that uses the bean tags to display the account bean properties. request.setCity("Lincoln").setCountry("USA"). address.accountType" scope="session"/> <bean:write name="account" property="accountDetail.

Must contain atleast 2 numbers error.username = Invalid UserName.number" scope="session"/> </br> <bean:write name="account" property="creditCards[1].properties title = Welcome Page error.password" />
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. Must be atleast 8 characters
To display the above property values in the browser. For instance.city" scope="session"/> property="address.username" /><br/> <bean:message key="error. It also specifies where the bean can be found using the scope attribute.state" scope="session"/> property="address.name" scope="session"/> <bean:write name="account" property="creditCards[0].accountType" scope="session"/>
Since we stored the bean with the name account in session scope.Chapter 16
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<br/> <bean:write <bean:write <bean:write <bean:write <br/> <bean:write name="account" property="creditCards[0]. it uses the following bean tag:
<bean:write name="account" property="accountDetail. We also used the message tags to read the properties from the following property file:
MyStruts.number" scope="session"/> <bean:write name="ssn" /> <h2>Messages from property files</h2><br/> <bean:message key="title" /><br/> <bean:message key="error.name" scope="session"/> <bean:write name="account" property="creditCards[1].country" scope="session"/>
This JSP uses the bean tags to retrieve the Account bean information. to retrieve the account type. we use the message tag as shown below:
<bean:message key="error. The property attribute is the one that specifies the complete path of the property.password = Invalid Password.addressLine1" scope="session"/> property="address. the above tag must also use the same name to access it.password" /> </P> </BODY> </html> name="account" name="account" name="account" name="account" property="address.

>classes>javac actions\*. Add the following action mapping in the action mappings section in the strutsconfig. 5.java C:/>JavaTraining>.java myweb/jsps/struts/BeanTagsDemo. This is the reason why we have the forward with the name bean in the configuration file as shown below:
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.properties
2.BeanAction"> <forward name="bean" path="/jsps/struts/BeanTagsDemo..do?page=bean
Notice the page parameter in the request. Steps to run the program 1.java myweb/WEB-INF/classes/beans/Account.Chapter 16
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The above tag will retrieve the property value of error.>..Message Resources --> <message-resources parameter="myprops. Start the server. .java myweb/WEB-INF/classes/beans/AccountDetail.>classes>javac beans\*. .. Let’s execute the program.java
4.java myweb/WEB-INF/classes/beans/CreditCard. Type the following URL in the browser
http://localhost:8080/myweb/customtags.java myweb/WEB-INF/classes/actions/BeanAction.>. .jsp myweb/WEB-INF/classes/myprops/MyStruts.. <action path="/customTags" type="actions. Compile the struts classes as shown below:
C:/>JavaTraining>. Save all the files as shown below
myweb/WEB-INF/classes/beans/Address.xml (Next to previous mappings)
<action-mappings> .jsp" /> </action> </action-mapings> <!-. The action class will use this parameter to select the view. . .MyStruts"/>
3.password from the property file and display it in the browser. .

<logic:empty> This tag is used to check the emptiness of a collection referenced by a JSP page variable or a bean property. Following are the most important tags in this category. conditions etc.jsp" />
The above URL produces the following result.
Logic Tags
These tags are used for basic operations in the JSP pages like iterations. The syntax for this tag is shown below:
<logic:empty name="" />
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. The body of the tag is evaluated based on the outcome. let’s look at how logic tags work. These tags are normally prefixed with the word “logic”.Chapter 16
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<forward name="bean" path="/jsps/struts/BeanTagsDemo.
Now that we understood how the bean tags work. These tags are very much similar to the JSTL core tags that we read in the previous chapter.

The syntax for this tag is shown below:
<logic:notEmpty name="" /> <logic:notEmpty name="" property="" />
<logic:equal> This tag as the name suggests will evaluate the body of the tag if the value of a page variable or bean property is equal to the specified value.
<logic:empty name=”accounts” property=”creditCards” scope=”session”> Some Content </logic:empty>
<logic:notEmpty> The behavior of this tag is exactly reverse of the above empty tag. %> <logic:empty name=”data”> Some content </logic:empty>
The above usage will evaluate the body of the tag if the collection referenced by data is empty Example 2: The following usage of the tag will evaluate the body if the collection referenced by creditCards property in accounts bean is empty. Its syntax is shown below:
<logic:equal name="" value="" /> <logic:equal name="" property="" value="" scope=”” />
Example 3:
<logic:equal name=”firstName” value=”John”> Some content here </logic:equal>
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<logic:empty name="" property="" scope="">
Example 1:
<% List data = new ArrayList(). This tag will evaluate the body if the collection is not empty.

See the code in listing 16. It uses the id attribute to reference every object in the collection.5 (LogicTagsDemo. Listing 16.tld" prefix="bean"%> <%@ taglib uri="/WEB-INF/struts-logic. we use the id value and read the properties as shown above.5. <logic:iterate> This tag is used to iterate over a collection of objects.Chapter 16
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The above usage of this tag will evaluate the body if the value in firstName variable is equal to John. Following is the syntax for this tag:
<logic:iterate id=”” name=”” />
Example 5:
<logic:iterate id=”customer” name=”account” property=”creditCards” scope=”session”> <bean:write name=”customer” property=”name” /> <bean:write name=”customer” property=”number” /> </logic:iterate>
This above usage of the tag will iterate over the creditCards collection in the account bean. Using the above logic tags let’s write an example.tld" prefix="logic"%> <html>
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.jsp) JSP page using logic tags
<%@ taglib uri="/WEB-INF/struts-html.jsp and use the account bean stored in the session class.tld" prefix="html"%> <%@ taglib uri="/WEB-INF/struts-bean. What we will do is. Inside the tag. </logic:equal>
The above usage of the tag will check the value of bankName property in account bean and then evaluate the content accordingly. use the same bean classes and action class of the previous example and simply create a new JSP file called LogicTagsDemo. This is one of the most widely used logic tag. Example 4:
<logic:equal name=”account” property=”bankName” scope=”session” value=”Swiss”> Thank you for choosing Swiss. The iteration will display all the credit card names and numbers stored in the account bean.

Following are the important ones.do" we use this tag as shown below:
<html:form action="welcome">
Notice that we need not specify the “. <html:form action=""> This tag is used to create a html form element.jsp" /> </action>
3.do?page=logic
The above URL produces the following result
HTML tags
The Struts html tags are used to represent the standard html elements in the JSP page. To submit a html form to an action mapping named "/welcome.jsp" /> <forward name="logic" path="/jsps/struts/LogicTagsDemo. <html:text property="" size="" maxlength="" />
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. Start the tomcat server and type the following URL
http://localhost:8080/myweb/customtags.do” extension. Though these tags work much the same way.BeanAction"> <forward name="bean" path="/jsps/struts/BeanTagsDemo. it’s a good practice to use them to be consistent. These tags are normally prefixed with the word “html”.Chapter 16
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<action path="/customTags" type="actions.

we use this tag a shown below:
<html:text property="firstName" size="20" maxlength="50" />
<html:textarea property="" rows="" cols="" /> This tag is used to create html text area with specified name.do?firstName=john&lastName=Smith
<html:select collection="" property="" /> This tag is used to represent a drop down box for selecting the items.put("lastName". %> <html:link action="register" name="queryString"/>
The above hyperlink gets manifested as a URL shown below:
http://localhost:8080/myweb/register. queryString."Smith"). and with maximum character limitation of 50. Example 1: To create a text field with the name firstName of size 20."john"). rows and columns. Example 3:
<% Map queryString = new HashMap().Chapter 16
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This tag is used to create html text field. queryString. we use this tag as shown below:
<html:textarea property="description" rows="5" cols="40" />
<html:link action=""
name="" />
This tag defines a hyperlink. Example 4:
<%
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.put("firstName". Example 2: To create a text area with the name description with dimensions 5 by 40. The name attribute is optional and if present must be a Map which will get converted to queryString.

the selected item in the drop down will be sent in the request with the property name book as shown below:
http://localhost:8080/myweb/select. we will create a form bean to collect the data. 2. This completes all the basic custom tags that we need to know to build JSP pages. Since we have a form. let’ get back to the real struts business. If all the data is valid. Struts support data validation through two mechanisms listed below: 1. This example also introduces you to data validation using form beans. we will build a JSP page that uses the struts custom tags and post the customer information to an action class. Following is what we will do in this example: 1. books. controller will send the formbean to the action class.add("Java").
Validation using FormBeans
The example that we are going to develop now will cover all the concepts that we learned so far.add("Perl Scriping"). The controller will use the form bean to store and validate the data. The customer form in the JSP will be submitted to an action class. books.Chapter 16
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ArrayList books = new ArrayList(). With this knowledge.add("C Programming"). In this example. Formbean Validation 2.
Struts Validation
Validating the form data is one of the common things we do in web applicatons. 3. The usage of this tag is explained in the later section. Struts Validator Let’s look at both the mechanisms one by one and see which one offers more flexibility. Send a request from the browser to access the JSP page.
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.do?book=Java
<html:errors /> This tag is used to display all the Struts errors. books. %> <html:select collection="books" property="book" />
When the form is submitted.

The validate attribute tells the controller to use the formbean and validate the data.Chapter 16
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CustomerDetails.required")).required")). new ActionError("error.lastName. The name attribute specifies the form bean to use. validate and input.do action as shown below:
<html:form action="customer">
The configuration file defines the customer action as shown below:
<action path="/customer" name="CustomerForm" validate="true" input="/jsps/struts/CustomerDetails. CustomerForm.add("lastName".jsp This page uses the struts html tags to build a customer form and posts the data to /customer. Usually. Here is the trick.jsp" / </action>
The above mapping has three important attributes namely name.CustomerAction"> <forward name="success" path="/jsps/struts/Confirmation. The input attribute tells the controller to forward to the specified page in the event of validation errors. new ActionError("error.jsp" scope="request" type="actions. } // Validation for valid values for SSN and Age
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. // Validate the fields in your form // Lastname and SSN are always required if ((lastName == null) || (lastName.java This formbean simply defines all the properties of the form along with the getters and setters.length() == 0)) { errors. } if ((ssn == null) || (ssn.ssn. To validate the data inside formbean.add("ssn". it should implement the validate() method as shown below:
public ActionErrors validate(ActionMapping mapping. the JSP page specified by this atribute will be the same page from where the form is submitted. HttpServletRequest request) { ActionErrors errors = new ActionErrors(). The important thing to understand here is how the controller uses the bean to validate the data.length() == 0)) { errors.

lastName.
error. in our case the message will be “LastName is required”. With the following property defined in the property file. new ActionError (“error.parseInt(ssn). }
The above code checks for lastName.
The constructor usually specifies the key defined in the property file whose value will be echoed when the validation fails.add("ssn_number". Since the validation is turned on using the validate attribute in the action mapping. } catch (Exception e) { errors.lastName.add("lastName".invalid")).required”).required")). } try { int age1 = Integer. new ActionError("error. the controller after populating formbean properties will invoke the validate() method.
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. } return errors. }
This method is a standard method for validating the data. If lastName is not populated. new ActionError("error.invalid")). it creates an action error object as. and does the following two things: 1.length() == 0)) { errors.Chapter 16
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try { int ssn1 = Integer. we need to add an ActionError object to the ActionErrors list as shown below:
if ((lastName == null) || (lastName.add(“lastName”.age. For every failed validation. then the user entered non-numeric data for SSN errors. } catch (Exception e) { // If it comes here.lastName.parseInt(age).ssn. the actual message from the property file will be displayed.lastName.required”). This method will return ActionErrors object which is a collection of ActionError objects. new ActionError("error.
new ActionError (“error.required = LastName is required
when the errors are displayed on the page. 2.add("age_number". The above action error will then be added to the ActionErrors list as
errors.

invalid = Age must be a 2 digit number
CustomerAction.jsp
Enter non-numeric data for Age and SSN and you’ll see the following result:
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. It finally selects the success forward which takes us to the confirmation page using the following forward definition:
<forward name=”success” path=”/jsps/struts/Confirmation.ssn.age. let’s display the errors at the top of the page by placing the above tag as shown below:
<BODY> <html:errors/> <P>Please fill in the following details</P>
Run the example by entering the following URL in the browser:
http://localhost:8080/myweb/jsps/struts/CustomerDetails. We simply need to add the following html errors tag in the jsp page where ever we want to display the errors. the controller will go back to the page specified by the input attribute in the action mapping which happens to be the same JSP page that submitted the form. we add the errors for other fields.required = LastName is required error. Take this part as assignment to implement the JDBC logic. Assuming that validation errors if any are returned back to the page. By the end of the validate() method. Since the formbean is performing four validations. Otherwise.required = SSN is required error. we will have four property messages defined in the properties file as shown below:
error.Chapter 16
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Similarly.ssn. then the form bean will be sent to action class.invalid = SSN must be a 10 digit number error. how should we display them? Very simple.
<html:errors/>
In our example.lastName.java The execute() method of this action class simply reads the formbean data and invokes the method to insert the data into the database.jsp”/>
There is one more thing we need to know. if the ActionErrors list is empty (no errors) .

Though formbeans give us the flexibility for custom validations. Now you know how to validate form data using formbeans. it has some serious drawbacks too. we need to have 100 formbean classes. right? This poses code maintenance problems and the project size also gets lengthy.
To overcome the above two problems. does struts have a solution by which we can completely eliminate the formbean classes and also eliminate the recompilation of the project due to form changes? Yes. 1. without wasting any time. The new type of formbeans use DynaActionForm class instead of ActionForm class to address the problems. Let’s see this negative side of formbeans. This is
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. we have a solution in the form of dynaformbeans. they all belong to ActionForm class because the formbean inherits from it as shown below:
public class CustomerForm extends ActionForm
Such formbeans suffer from the above two problems. Formbeans using DynaActionForm class If you noticed the formbeans we wrote until now. then the formbean properties must be changed accordingly. which requires recompilation of entire project. let’s see what this is all about.Chapter 16
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Congratulations.
2. if the web application has like 100 html forms. If the html form undergoes changes by adding or removing fields. Since a single formbean is associated with a single html form. So.

action class first casts the formbean into the appropriate class and
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. So.lang.validator.validator. what about validation? Clearly. The solution is XML based validation using Struts Validation framework.String" /> <form-property name="age" type="java. struts is there for our rescue.String" /> <form-property name="middleName" type="java. Using DynaActionForm we can eliminate all the formbean classes by declaring the formbean properties in the struts configuration file (struts-config.String" /> <form-property name="lastName" type="java. if we want to do data validation.String" /> <form-property name="ssn" type="java. it will be defined as shown below:
<form-property name="ssn" type="java.DynaValidatorForm"> <form-property name="firstName" type="java.lang.xml) itself. we no longer have to recompile the project in the event of changes. This is the beauty of Struts. There are two solutions.String" /> </form-bean>
There are two important things we need to understand with the above definition.DynaValidator Form and we need to use nested <form-property> tags to define the bean properties.String" />
The above defines firstName as a String.String" /> <form-property name="city" type="java.String" /> <form-property name="country" type="java.lang. Following is how the formbean definition for a customer form looks like:
<form-bean name="dynamicCustomerForm" type="org.lang.Integer" />
Though the above definition solved the two problems. By moving the entire formbean definition into the xml itself. we did not move the validation logic in the validate() method to the xml.lang. what should we do? Good question.struts.lang.apache.struts. The formbean definition will go right in the form beans section of the xml file. The type attribute should always be org. right? Using this solution brings us back to square one. Every time we have a problem. One solution is create a class and just define the validate() method. if you want to define a property to store numbers. right? So. Now that we know how to define a dynaformbean in the configuration file. this is definitely not the solution.lang. Similarly.lang. What? a new class again? This is what we want to eliminate.Chapter 16
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why we call them as dynaformbeans. We just have to restart the server. This element should also define the class of the property using the type attribute as shown below:
<form-property name="firstName" type="java. we need to know how to read the data from this bean in the action class.String" /> <form-property name="state" type="java.lang.lang. If you recall from the previous example.apache.

Listing 16. Let’s stop this theory crap and look at an example using the dynaformbeans. try { // We have a form bean in this case DynaValidatorForm bean = (DynaValidatorForm) form.action.apache.ActionForm. org.struts.DriverManager.7.apache. We do almost exactly the same with dynaformbeans too.PreparedStatement.HttpServletRequest.
As you can see from the above code.struts.get("firstName"). String middleName = bean. we cast the form to DynaValidatorForm class and use the get methods passing the property names to read the data.Chapter 16
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then invoke the getter methods to read the data.getAttribute("Name") %> for visiting the WebSite </h3>
Listing 16. but post it to a new action class that uses dynaformbean.action. HttpServletResponse response) throws Exception { String nextPage = "success".get("middleName").action.Connection. See the code in listing 16.servlet.java) Action class using dyna form beans
package actions.apache.ActionForward. HttpServletRequest request. import javax.sql.ActionMapping.Action.7a (ConfirmationPage2.struts. org.http.http.HttpServletResponse. import java.7b (DynamicCustomerAction.servlet. import javax.apache. In this example we will use the same JSP code from the previous example. org. import import import import import org. String firstName = (String) bean.struts.
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. String firstName = bean.
public class DynamicCustomerAction extends Action { public ActionForward execute(ActionMapping mapping. Following is how the code looks like in the action class:
DynaValidatorForm bean = (DynaValidatorForm)form.struts. import java.validator.sql.get("firstName").sql.jsp) Confirmation page
<%@ page session="true" %> <h3> Thank you <%= session. ActionForm form. org.DynaValidatorForm. import java.action.apache.

http://localhost:8080/myweb/jsps/struts/CustomerDetails.jsp page created in the previous example. Since this action mapping uses dynaformbean.jsp. change the form action as shown below to point to the above action mapping. The JSP will post the form data to dynaCustomer action. For now we commented the method to insert the data into database. the controller will populate the bean properties with the form data and passes it as an argument to action class. and type the following URL in the browser. This JSP will read the Name field from the session and displays a greeting to the user. In the CustomerDetails. It also defines a forward to Confirmation2. 4.jsp
The above URL will produce the following result:
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.DynamicCustomerAction "> <forward name="success" path="/jsps/struts/Confirmation2.jsp"/> </action>
Notice that our new formbean name is “dynaCustomerForm” which uses the dynaformbean defined in Step 2. If you have the table ready. Start the server. The action class then selected the success forward which forwards it to confirmation page defined in the action mapping. then you can uncomment this code to store the data in the table.
<html:form action="dynaCustomer">
5. The action class simply reads all the data and stores the Name of the customer in the session.Chapter 16
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type="actions.>myweb>WEB-INF>classes>javac actions\*. Compile the action class as shown below
c:/>JavaTraining…….java
Let’s see how the code works.

Struts Validator Framework
Struts validation framework is a built in validation model for data validation.Chapter 16
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Now that we know how to use dynamic formbeans for capturing the form data. Here is one thing that you should always keep in mind. the only solution left is the “Validation Framework”.
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. it’s time to understand how we can do data validation using the same. Since we already ruled out the usage of additional class. Let’s see what this is. This framework is based on XML and offers several benefits over the regualar formbean validation. XML based utilities are always powerful and offers immense flexibility.

let’s see how this element typically looks like.util.xml 2.commons.apache. but usually have just one global element.validator.apache.action. The validation.HttpServletRequest"
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. This validator completely eliminates the coding of formbean classes and declaratively configures the validation logic in the XML files.ActionErrors. The validator element supports six attributes as shown below:
<validator name="required" classname="org.commons.xml is a standard XML file that comes along with the Struts distribution.
validation-rules. This element can have one or more global elements.xml file is the one that we create and declare all the validations our application needs. org.StrutsValidator" method="validateRequired" methodParams="java.servlet.Object.validator. Following is the basic structure of this XML file:
<form-validation> <global> <validator/> <validator/> </global> </form-validation>
As shown above.struts. the root element of this XML document is the form-validation.xml The validation-rules.apache. validation-rules. This file defines all the standard validation functions that are most widely and commonly used in typical web applications. There is absolutely no limit on the number of validation rules it can have.apache.Field.ValidatorAction. javax. Seldom web applications go beyond the validation rules defined in this file.struts.lang. Let’s understand the usage of these two XML files in detail.xml
This XML as I said before comes along with the Struts distribution. The global element will have several validator elements each defining a validation rule.Chapter 16
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The validation framework is created by David Winterfeld and it now integrated into the Struts framework as part the distribution. If the standard rules don’t meet your requirements. The validator element forms the heart of this XML. This framework uses the following two XML files for validation: 1. you are free to add new validation rules. So. org. org.http. validation.

required"
Therefore.Chapter 16
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msg="errors.2 Validator Attributes Attribute name classname Description A unique name that identifies the validation rule.mask” />
Following table lists the purpose of each attribute. we need to define the above key in the properties file with the actual error message as shown below:
errors. This represents a comma delimited list of parameters to the above method. don’t get scared by the length of the XML file. StrutsValidator which is a standard built-in struts class. This attribute represents the name of the method in the above class that does the validation. So. We sincerely need to copy paste them into the property file. This represents the key in the property file whose value should be used in the event of validation error This attribute defines names of other validation rules that should be fired before firing the current rule. We just have to copy paste them into our property file. Following are the validation keys used by the validation rules in this XML file.required = Last Name is required. In this case. for the validation rule to work. It takes five parameters.
Every validation rule will have its own message key and therefore we need to define each an every key in the property file.
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.struts. Once such method is validateRequired. the name if the rule is required This represents the class that contains the actual validation logic. the XML file defines all the message properties in the comment section. The good thing is. Table 16.required" depends=”numeric. This method does the required validation for a form field.apache. This will always be org. We can specify more then one dependent rules here using a comma delimiter.
method
methodParams msg depends
This validation-rules XML file will define several such validator elements one for each validation rule.util. All the validation rules in this XML file uses the msg attribute that defines the property key to get the actual error message as shown below:
msg="errors.

Don’t worry about the numbers in the message text. errors. remember them as arguments to the message.xml.minlength={0} can not be less than {1} characters.required={0} is required. errors. there will be a corresponding method defined in the StrutsValidator class. ssn) Used for validating minimum length for a field Used for validating maximum length for a field Used for valid email addresses Used for valid phone numbers
This is all you need to know about the validation-rules.xml file. Following table lists the rules and corresponding methods.date={0} is not a date. errors. errors. Table 16. errors.3 Validation functions and equivalent methods Validation Rule Name required date creditcard integer minLength Range email Method in the StrutsValidator validateRequired() validateDate() validateCreditCard() validateInteger() validateMinLength() validateRange() validateEmail()
The most important validation rules that we frequently use are: Table 16.Chapter 16
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errors. We’ll look at their significance in the later section. errors. the validation.integer={0} must be an integer. let’s look at the second XML file.creditcard={0} is an invalid credit card number. For every validation rule in the XML file.range={0} is not in the range {1} through {2}.email={0} is an invalid e-mail address. errors.4 Important Validation functions Validation Rule Name required integer minLength maxLength email phone Description Used for required validation for a field (Ex: LastName) Used for numeric validation for a field (Ex: age. errors.invalid={0} is invalid. For now.
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. Now.maxlength={0} can not be greater than {1} characters.

xml are required to be fired for various formbean fields.xml
This file defines which validation rules in the validation-rules.range”> <arg0 key=”customerForm.integer”> <arg0 key=”customerForm. The XML structure for this file is shown below:
<form-validation> <global> <constant/> <constant/> -----</global> <formset> <form> <field/> <field/> ---</form> <form> <field/> <field/> </form> <formset> </form-validation>
As you can see from the above xml fragment. The form element is the most important one that specifies which validation rules need to be fired for various form fields. the root form-validation element has two child elements namely global and formset.ssn”/> </field> <field property=”age” depends=”required. The global element defines one or more constant elements and formset element defines one or more form elements.lastName”/> </field> <field property=”ssn” depends=”required.Chapter 16
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validation.age”/> <arg1 name = "range" key="$[val:limit1]" resource="false"/> <arg2 name = "range" key="$[val:limit2]" resource="false"/> <var> <var-name>limit1</var-name> <var-value>10</var-value> </var> <var> <var-name>limit2</var-name> <var-value>20</var-value></var> </field> </form>
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. Consider the following form element
<form name=”customerForm”> <field property=”lastName” depends=”required”> <arg0 key=”customerForm.

required={0} is required.required={0} is required. Look at the following field definition for SSN. Therefore. errors.lastName”/>
where customerForm. then it fires the integer validation that checks whether the value entered in the field is a number or not.ssn”/> </field>
The only difference with this field is that we added integer validation in the depends attribute.integer”> <arg0 key=”customerForm. If a value is entered for this field.
<field property=”lastName” depends=”required”> <arg0 key=”customerForm.lastName = Last Name
With all the above.lastName”/> </field>
This field uses the required validation as specified by the depends attribute. Following field defines the validation for lastName. If it is not a number. For this field.lastName is the property key defined in the property file as shown below:
customerForm. Let’s look at the field elements one by one.
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. the error message produced for this field is “Last Name is required”.
<field property=”ssn” depends=”required.Chapter 16
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The above form element defines validations for three form fields in a formbean named customerForm whose names are lastName. then it generates an error message. we need to pass the field name for 0th argument as shown below:
<arg0 key=”customerForm. ssn and age respectively. Following two properties are used for this validation:
errors. This validation uses the following property in the property file to display the error message:
errors. the struts validator first performs the required validation.integer={0} must be an integer.
In the above message “{0}” represents the 0th argument which should be replaced with the actual field name.

age”/> <arg1 name = "range" key="$[val:limit1]" resource="false"/> <arg2 name = "range" key="$[val:limit2]" resource="false"/> <var> <var-name>limit1</var-name>
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. if I have three arguments as shown below. Trick: If the “depends” attribute uses say.range”> <arg0 key=”key1”/> <arg1 key=”key2”/> <arg2 key=”key3”/> </field>
and if the following keys are defined in my property file as.
2.range={0} is not in the range {1} through {2}. then follow a two step process to determine the number of arguments to pass: 1. From the above messages it is (2 + 1) = 3.range”.integer.
<field property=”test” depends=”required. 0th argument will be used by first three messages. How many arguments do we need to pass for the above two messages? One or two? Good. We have to pass only ONE argument. The total number of arguments is the largest argument number + 1. errors.
key1 =Age key2=20 key3=30
then the possible three error messages will be
Age is required Age must be integer Age is not in the range 20 through 30
Sometimes.Chapter 16
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Let me ask you a question.integer. and 1st and 2nd arguments will be used by third message. This is because each of the above messages only need one argument.integer={0} must be an integer.required={0} is required. users prefer to define the actual values in the XML itself as opposed to defining them in property file. Identify the error messages for the three functions. This gives us the following
errors. errors. if we want to define values 20 and 30 in the field definition itself. So. we can do it as shown below:
<field property=”age” depends=”required. three validations as “required. In the above example.range”> <arg0 key=”customerForm.

for range validation (name=range).0" encoding="ISO-8859-1" ?> <!DOCTYPE form-validation PUBLIC "-//Apache Software Foundation//DTD Commons Validator Rules Configuration 1.xml) Validation xml file
<?xml version="1. ssn and age fields: lastName: required ssn: required. Now look at 1st argument.8 Listing 16. range Take a look at the code in listing 16. I think with all the above theory crap. If at any time you want to declare variable values that should be plugged in the error message.age" /> <arg1 name="range" key="$[var:minLength]" resource="false" /> <arg2 name="range" key="$[var:maxLength]" resource="false" /> <var>
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.To conclude. integer age: required. let’s do an example and you’ll be an expert.org/commons/dtds/validator_1_0. integer. use the value of limit1 variable (key="$[val:limit1]"). you now know how the struts validator works.apache. We will use the previous dyna formbean example and add the following validations for lastName.ssn" /> </field> <field property="age" depends="required. you can do so using the above convention. It says.0//EN" "http://jakarta. I am sure the picture is clear.lastName" /> </field> <field property="ssn" depends="required.Chapter 16
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<var-value>10</var-value> </var> <var> <var-name>limit2</var-name> <var-value>20</var-value> </var> </field>
In the above snippet. don’t use the property file (resource = false).range"> <arg0 key="customerForm.8 (validation.integer"> <arg0 key="customerForm. we defined to variables namely limit1 and limit2 with values 10 and 20.dtd"> <form-validation> <formset> <form name="dynamicCustomerForm"> <field property="lastName" depends="required"> <arg0 key="customerForm.

xml and validation. Once the above field definitions are definied.xml
(Note: The rules xml file comes with struts installation.date={0} is not a date. errors. errors. customerForm.byte={0} must be a byte.ssn = SSN customerForm. Just copy it) 2.email={0} is an invalid e-mail address.creditcard={0} is an invalid credit card number.maxlength={0} can not be greater than {1} characters.short={0} must be a short. errors.long={0} must be a long.range={0} is not in the range {1} through {2}. errors. we are ready to execute the program. errors.lastName = Last Name customerForm.xml file in
/myweb/WEB-INF/validation-rules. Steps to run the program 1.float={0} must be a float. errors.invalid={0} is invalid.minlength={0} can not be less than {1} characters. errors. errors.age = Age
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. errors.required={0} is required.
errors.integer={0} must be an integer. errors. Save the validation-rules.xml /myweb/WEB-INF/validation.xml file.double={0} must be a double. errors. errors.Chapter 16
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<var-name>minLength</var-name> <var-value>10</var-value> </var> <var> <var-name>maxLength</var-name> <var-value>20</var-value> </var> </field> </form> </formset> </form-validation>
The above validation xml defines validation rules for a form named dynamicCustomer Form as shown below:
<form name="dynamicCustomerForm">
The form name defined by this element must be exactly same as the formbean name used in the struts-config. Add the following properties in "MyStruts. errors.properties" file.

Just copy and paste it as we only do it once. You don’t have to remember the syntax of this element. You’ll see the errors
Last Name is required SSN is required Age is required
2.jsp" type="net. SSN and Age.Chapter 16
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3. Struts validation framework by default will not be enabled. with invalid SSN and Age. Ok. Add it next to the message resources section
<! . Add the following xml snippet in struts-config.struts. Enter 40 for Age and you’ll see
Age is not in the range 20 through 30
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.xml. Step 3 deserves some attention. we also need to specify the locations of the two XML files validation.xml" /> </plug-in>
4. Update the previous action mapping to validate the data as shown below:
<action path="/dynaCustomer" name="dynamicCustomerForm" validate="true" input="/jsps/struts/CustomerDetails.xml.DynamicCustomerAction"> <forward name="success" path="/jsps/struts/Main.xml to turn on the validation framework. If you noticed in the plugin declaration.xml and validation-rules.nwie. 1. You’ll see the errors
SSN must be integer Age must be integer
3..actions. We need to ask the struts framework to enable it by defining it as a plugin as shown above.validator. Now that we completed all the steps.. In this example. Don’t enter lastName.jsp
Try the following cases.struts.ValidatorPlugIn"> <set-property property="pathnames" value="/WEB-INF/validator-rules. start the server and test the following URL
http://localhost:8080/myweb/jsps/struts/CustomerDetails./WEB-INF/validation. we are using all the code from the previous example.Plugin Details . Enter lastName.> <plug-in className="org.jsp" /> </action>
In the above process.apache.

we can display error messages using JavaScript dialog boxes.">
2. Also add the following line in the JSP
<html:javascript formName="dynamicCustomerForm"/>
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.Chapter 16
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There is one more interesting feature with the struts validator framework. and I am sure you’ll like it. Follow the process below: 1.
Java Script Validation
This is an extension to the validator framework to display the error messages using Javascript dialog boxes.jsp page as shown below
<form action="dynaCustomer" onSubmit="return validateDynamicCustomerForm(this). With simple modifications. Let’s do this. Update the CustomerForm.

Now run the JSP again with the same URL as before.Chapter 16
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Since our formbean name is dynamicCustomerForm.
In Step 2. the formbean name should start with an uppercase letter and the function should be terminated by a semicolon as shown below:
onSubmit="return validateDynamicCustomerForm(this). No changing case here. we need to specify the onsubmit attribute with the JavaScript function name using the following syntax: validate <form bean Name> (this). Using this framework. ok. In the above function name. we need to use one of the html tags with the formName attribute specifying the formbean name. in step 1. There is one more utility framework in
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. and you’ll see the error messages as shown below:
This completes all the details of Struts validator framework. we can completely eliminate the formbean classes and make use of xml for data validation which offers more flexibility and ease of use.

xml" />
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. The smaller pages are referred to as tiles. Using struts tiles framework is very simple. There is a standard and simple process to follow as depicted below: 1. navigation bars etc. Let’s see what this framework has to offer. footers.xml. A typical web page can be viewed as composite page built from several small pages (tiles) arranged according to a fixed layout called master layout. The details of this file are deferred to a later section.apache. most of the pages will have certain things in common like headers. we need to configure the tiles plugin just like we plugged in the struts validator in the configuration file. The name of this file can be anything.TilesPlugin"> <set-property property="definitions-config" value="/WEB-INF/layout-tiles-defs. but we usually name it as layout-tiles-defs. This is cool. It is used for web page layout. To use Struts tiles framework. Create smaller JSP pages that will be used to build the composite page. The tiles framework like the validation framework uses an XML file to define the master layouts and tile information.
<plug-in className="org.xml file to turn on the tiles framework. By using master layouts to build composite pages. Every tile will have a unique name to identify it.
Tiles Framework
In real world applications there will be several web pages. we can increase the flexibility of the web pages. Struts tiles framework supports building such composite pages using master layouts. if you want to use a different layout for the page. 2.xml" /> <set-property property="moduleAware" value="false"/> <set-property property="definitions-parser-validate" value="true"/> </plug-in>
If you notice the above plugin element.tiles.struts. Create a master layout JSP page with different tiles.Chapter 16
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Struts just like the validator framework. Following is the plugin information that we will be adding to the struts-config. it takes the location of the tiles XML file as a parameter as shown below:
<set-property property="definitions-config" value="/WEB-INF/layout-tiles-defs. and all your web pages will take the new form automatically. but this is for a different purpose. all you have to do is change the master layout. right? You don’t have to manually change each and every page to adjust to the new layout. If you noticed carefully. Tomorrow.

<tiles:definition id=”” page=””/> This tag is used by the composite page to specify the master layout to use. Listing 16. At the beginning of the chapter.9a (masterLayout. bean tags and logic tags. These are referred to as tiles tags.Chapter 16
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Once we added the above plugin in the configuration file. we are ready to build applications using tiles. <tiles:insert attribute=””> This tag is used to insert the tile in the master layout.9.
<%@ taglib uri="/WEB-INF/struts-tiles. Let’s see a quick example to build a composite JSP page based on the following master layout. There is one more category of tags which is exclusively used with tiles framework. we learned three categories of struts custom tags namely html tags.
Header Body
Nav
Footer
Take a look at the code in listing 16. The above three tags are more than enough to develop JSP pages using tiles.tld" prefix="tiles" %> <html> <body> <TABLE border=1> <TR> <TD colspan=2 width=1001 height=93><div >
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. Following are the important ones.jsp) Master layout JSP page. The tags are normally prefixed with the word “tiles”. <tiles:put name=”” value=””/> This tag is used by the composite page to associate the actual tile page in to be used in the master layout.

You can build a fancy one </h1> <br/><h3> <i>Copyright HeloWorld Inc.Chapter 16
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<tiles:insert attribute="header" ignore="true" /> </TD> </TR> <TR> <TD width=227 height=604> <tiles:insert attribute="navigator" ignore="true" /> </TD> <TD width=774 height=604> <tiles:insert attribute="body" ignore="true" /> </TD> </TR> <TR> <TD colspan=2 width=1001 height=107> <tiles:insert attribute="footer" ignore="true" /> </TD> </TR> </TABLE> </body> </html>
If you look at the JSP code.jsp
<html> <body> <h1> This is a Footer. You can build a fancy one </body> </html>
</h1>
Footer. When we actually build the composite page using the master layout.
Header.9b Individual tile pages.
<tiles:insert attribute=”header” ignore=”true”/>
As you can see from the above tag. we used the tiles insert tags to define various tiles in the master layout . every tile will have a unique name like header in the above tile. 2006 </i></h3> </body> </html>
Body.jsp
<html> <body> <h1> This is a Body. Listing 16.jsp
<html> <body> <h1> This is a Header. Let’s now build the smaller pages also referred to as tiles. we’ll associate the header tile with the actual JSP content. You can build a fancy one
</h1>
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. Following is how this tag is used.

Header. Uses the masterLayout.Chapter 16
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</body> </html>
Navigator.9c A composite JSP page. You can build a fancy one </body> </html>
</h1>
The above code forms the smaller pages or tiles that be used in building the composite page.jsp" flush=”true”> <tiles:put <tiles:put <tiles:put <tiles:put name="header" value="/jsps/struts/Header. There are two important things it does as listed below: 1.jsp
Start the server and type the following URL to access the Composite page:
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.jsp using the tiles:insert tag. 2.jsp"/> name="footer" value="/jsps/struts/Footer.jsp"/>
</tiles:insert>
Look at the above composite page. Save the JSP files as
myweb/jsps/struts/layouts/masterLayout.jsp myweb/jsps/struts/Navigator. Listing 16.jsp myweb/jsps/struts/Footer. Associates the actual tile pages to different tiles in the master layout.jsp myweb/jsps/struts/Body. For instance.
<%@ taglib uri="/WEB-INF/struts-tiles. That’s it.jsp page will replace the tile identified by header in the master layout page.jsp myweb/jsps/struts/Header. Let’s now build the composite page.jsp"/> name="body" value="/jsps/struts/Body.tld" prefix="tiles" %> <tiles:insert page="/jsps/struts/layouts/masterLayout.jsp myweb/jsps/struts/Composite1. Steps to run this example 1.jsp
<html> <body> <h1> This is a Navigator.jsp"/> name="navigator" value="/jsps/struts/Navigator.

But still.jsp"/>
</tiles:insert>
Of course.jsp"/> name="body" value="/jsps/struts/Body. Let’s say want to build 10 composite pages using the same master layout.jsp.Chapter 16
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http://localhost:8080/myweb/jsps/struts/Composite1. the smaller pages got assembled and formed up a composite page using the mater layout.jsp" flush=”true”> <tiles:put <tiles:put <tiles:put <tiles:put name="header" value="/jsps/struts/Header. Though the above example succeeded in building composite page. do we really need to create a separate JSP just for the above definition? If there a solution where we can store all the definitions in one file and permanently eliminate creating composite JSP pages then I guess it is the best solution.jsp"/> name="footer" value="/jsps/struts/Footer. the content is just the tile definition as shown below:
<tiles:insert page="/jsps/struts/layouts/masterLayout. I need to physically build ten different JSP files as Compsite1. just like we eliminated the formbeans classes using
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. Composite2. each will have it own definition though.jsp"/> name="navigator" value="/jsps/struts/Navigator. I see small maintenance problem. right? If you carefully observe the composite page.jsp
The above URL produces the following result:
As you can see from the above result.jsp and so on.

since we had the Composite1. we called it directly from the browser to see the response. The name attribute identifies the composite page.dtd"> <tiles-definitions> <definition name="composite1" path="/jsps/struts/layouts/masterLayout. the question is. Earlier. Now you can permanently delete the Composite1. We can store all the tiles definitions for all the composite pages in a single XML file and then have the tiles framework assist us in selecting a suitable definition. So the question is. This XML is nothing but the layout-tiles-defs.xml fine that forwards the request to the tiles definition as shown below:
<action path=”/comp1” forward=”composite1” />
Once we have the above mapping. Now that we deleted it and moved the definition to an XML file with the name composite1.jsp" /> <put name="body" value="/jsps/struts/Body.jsp"> <put name="head" value="/jsps/struts/Header. After creating this XML file.xml.apache. That’s it. The body elements will use the put element instead of tiles:put tag.jsp" /> </definition> </tiles-definitions>
Can you notice the difference? We use the definition element with two attributes. do we have such solution? Thankfully Yes. The definition in the XML will look as shown below:
<!DOCTYPE tiles-definitions PUBLIC "-//Apache Software Foundation//DTD Tiles Configuration//EN" "http://jakarta. This logically means we can indirectly call it.org/struts/dtds/tiles-config_1_1.do
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. right? This is done by simply writing an action mapping in strutsconfig.jsp"/> <put name="navigator" value="/jsps/struts/Navigator.jsp"/> <put name="footer" value="/jsps/struts/Footer. You can’t call the definition directly from the browser. we need to copy paste the tiles definition we had in the JSP page in to the XML file and make few modifications. we will call the action from the browser as shown below:
http://localhost:8080/myweb/comp1.jsp file. Let me ask you a question here. and the path attribute specifies the master layout page to use for that definition.jsp file. how can we call this definition which is in the XML file from the browser? The answer is simple.Chapter 16
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dynaformbeans.

xml file
<action path=”/comp1” forward=”composite1” />
3. we simply use extends attribute as shown below:
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. You can also define the tiles definition in the action forwards section as shown below:
<forward name=”success” path=”composite1”/>
Implement the following steps to use XML definition for the previous example: 1. right? The best solution will be to reuse the put elements from the composite1 definition.jsp" /> <put name="body" value="/jsps/struts/Body1.struts. all we changed is the name attribute and the body element with the new JSP page Body1.
<plug-in className="org.TilesPlugin"> <set-property property="definitions-config" value="/WEB-INF/layout-tiles-defs. The other three put elements are duplicated in my definition.
http://localhost:8080/myweb/comp1. Though the above definition works perfectly fine.tiles.jsp"/> <put name="footer" value="/jsps/struts/Footer.jsp"> <put name="head" value="/jsps/struts/Header. Copy and save the above XML in the following file as
/WEB-INF/layout-tiles-defs. Start the server and type the following URL. I see some redundant code.jsp"/> <put name="navigator" value="/jsps/struts/Navigator. navigator but different body.xml file next to the validator plugin.xml" /> <set-property property="moduleAware" value="false"/> <set-property property="definitions-parser-validate" value="true"/> </plug-in>
3.do
Let’s say I want to build another composite page that should have the exact same header. You’ll see the same result.jsp. Normally my new definition in the tiles xml file will look as shown below:
<definition name="composite2" path="/jsps/struts/layouts/masterLayout. footer.Chapter 16
Struts
Tiles Framework
The action mapping will then forward the request to the tiles definition named composite1 from the tiles XML file.xml
2. To do this.jsp" /> </definition>
If you observe the above definition. Add the following tiles plugin in the struts-config. Add the following action mapping in the struts-config.apache.

Though we can have all the web pages in the universal language “English”. We now arrived at the last and final concept. This way of writing definitions is what we normally do in real world applications. right? So.jsp"/> </definition>
All we need to do is remove the path attribute.xml file as shown below:
<action path=”/comp2” forward=”composite2” />
4. Now you can also remove the three duplicate put elements.
http://localhost:8080/myweb/comp2.do
This completes all the things we need to know in tiles framework. The key idea behind internationalization feature is something called resource bundles. are you not losing customers and business? So. internationalization. You’ll love this one. let’s see how we can tune our web application to run in different country say France in French language. 2. As a business person. and add extends attribute with the name of the definition from which it should inherit the tiles. some people prefer their regional languages.jsp
3. Create a new Body2. Add a new action mapping in the struts-config.jsp how ever you want and save it as
myweb/jsps/struts/Body2.Chapter 16
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<definition name="composite2" extends=”composite1” > <put name="body" value="/jsps/struts/Body1. This is tiles definition inheritance in which one definition inherits tiles from other definitions. Start the server and type the above URL. A resource bundle is nothing but a collection of several property files. Implement the following steps to create a new composite page: 1. Copy the above composite2 definition into the “layout-tiles-defs. Let’s say we are trying to build a global web application. if you only build the application in one language.
Internationalization
Internationalization feature in Struts allows your web applications to run in different languages. Trust me.xml” file. our goal is to reach all possible customers all over the world. Java and Struts uses the following two things to achieve internationalization:
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.

I will have three property files as shown below:
MyStruts_en. it’s a bundle of resources and resources are nothing but the property files that have messages based on the locale of the country. Every locale will have its own property file as shown below:
MyStruts_en. but the values will be in different language as shown below: MyStruts_en.properties
Every property file will have the same keys defined in English.Russian
Resource Bundles
As the name suggests.properties
error. Locale 2. So. one per locale or language.properties
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. French and German.username= User Name is Invalid
MyStruts_fr.English fr .properties MyStruts_fr. Following are some of the standard locales: en . if I want my web application to run in English.properties
Notice the names of the property files.properties MyStruts_de.properties MyStruts_fr.properties MyStruts_de.Chapter 16
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1.Spanish in US ru . They must end with an “_locale”.France de – German es_US . Resource Bundles
Locale
This is a class in Java library that specifies the locale of the country in which the application is running.
Struts and Internationalization
The trick to build universal web applications is to define the same messages in several property files.

It’s just some crap I used to simulate :-) Once we defined the above property files.age = Age errors.age. Save this file as myweb/WEB-INF/classes/myprops/MyStruts_fr.invalid = Invalid Da Length for un SSN error.username = Invalid Da Le User Nam error.xml (No “_locale” extension)
<message-resources parameter="myprops.
error. Make sure the following message definition is present in the struts-config.ssn.range={0} is not in the range {1} through {2}.ssn.required = LastName is required <br/> error.
Notice that the keys will be exactly same. What we will do is create some property files and run our previous examples to see different messages.ssn = SSN customerForm.required={0} la de requiro errors.properties
error.
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.Chapter 16
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error. Let’s write a small application that runs in French language.properties” and copy the following properties.username= Usero Un Invalido
MyStruts_de. Age Da La must Na Numeric.properties 2.username= La De User Ine
(The above messages are not really French and German texts.integer={0} integero un presento errors.required = SSN is required</br> error.invalid = Invalid Na Value. but the values will be different. Struts will smartly use the messages based on the country in which the application is running. Change your system locale to French as indicated below:
Start->Settings->Control Panel>Regional And Language Options
Select the language as French(France) in the drop down and click OK. customerForm.lastName.lastName = Last Name customerForm. Create a property file named “MyStruts_fr.password = Invalido Un Passwordino error. Steps to implement 1.MyStruts" />
3.

From the developer standpoint all we need to do is create a new property file and store the messages in different language.
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. Now go back and change the language back to English. I am sure you now figured out why Struts has become a de facto standard for J2EE based web applications. You are now an expert in building web applications and let me say this. This is the beauty of Struts.
Summary
Struts is an open source framework for building web applications.jsp
The above URL will produce the following result
Look at the error messages now. Start the server and run the dynaformbean example with the following URL
http://localhost:8080/myweb/jsps/struts/CustomerDetails. Struts is based on Model View Controller architecture. This completes all the concepts we need to know in Struts. Do you like them? I am sure you do. you are now a professional Struts.Chapter 16
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Summary
4. The Struts framework will take care of the rest. The major components in Struts are the formbeans and action classes. JSP or even better a J2EE developer.

Bean tags are used to work with JavaBeans and their properties. Struts messages are usually defined in property files which allow international -ization of the application. Struts includes several built-in custom tags. This framework use XML to define tiles definitions. These are categorized into html. Which of the following class represent controller component in struts? a) ControllerServlet b) ActionServlet 3. conditions etc in the JSP page. textfields etc. Dynaformbeans eliminate the formbean classes by defining the form properties in the configuration file itself. Which of the following design is used by Struts framework? a) MVC1 Design b) MVC2 Design 2. bean.xml. Logic tags are used to general purpose operations like iterations. All the Struts components are defined in a configuration file named strutsconfig. Which of the following class is used to store the form data? a) Action b) ActionForm 4. This framework also supports messages displayed using Java Script dislog boxes.
Time to play 50-50
1. Action classes are used to process the requests. Tiles tags are used for page layout. Struts tiles framework is used for building composite pages using master layouts.Chapter 16
Struts
Time to play 50-50
Formbeans represent the HTML form data and also used to validate the data. Which of the following class is used to process the request?
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. Struts validator framework is an XML based framework for validating the form data. HTML tags are used to create standard html elements like forms. Dynaformbeans use struts validator to validate the form data. logic and tiles tags.

xml b) struts-config-xml 7.Chapter 16
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Interview Questions
a) Action b) ActionForm 5. Which of the following eliminates writing form bean class? a) ActionForm b) DynaActionForm 10. Which of the following file is used to configure struts applications? a) web. Which of the following element is used to define global forwards in the configuration file? a) <global-forwards> b) <action-mappings> 8. Which of the following component is used to validate the form data? a) ActionForm b) Action 6. Which of the following element is used to define the formbeans in the configuration file? a) <action-mappings> b) <form-beans> 9. Which of the following element is used to define action classes in the configuration file? a) <action-mappings> b) <actions>
Interview Questions
Question: Which design does Struts framework use? Answer: Model View Controller (MVC) design.
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.

Plugins (Validator and Tiles).
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.xml Question: What are the various elements defined in the configuration file? Answer: Form beans. Question: List the important Struts components. Action classes. Question: What is the name of the configuration file used in Struts? Answer: struts-config. Form beans. Action Mappings. The next few chapters are smaller ones and should be a cakewalk once you arrived at this point. Action Mappings. a JSP page acts as both View and Controller component while in MVC2 architecture. I am sure you enjoyed this chapter. Question: What is the name of the XML file in which all the validation rules are defined? Answer: validation-rules. Question: What is the difference between a local forward and a global forward? Answer: A local forward can be used by a single action class where as global forwards can be used by all the action classes. Action Forwards. Action Errors etc. Global Exceptions. Global Forwards. Message resources.xml This completes everything you need to know about Struts. Answer: Request Processors.Chapter 16
Struts
Interview Questions
Question: Explain how MVC design works? Answer: Refer to page 447. servlet is used as controller and JSP is used as View component. Question: What is the difference between MVC1 and MVC2 architecture? Answer: In MVC1 architecture.

jar.Chapter 17
Java Messaging Service (JMS)
Chapter Goals
Chapter 17
Java Messaging Service (JMS)
This chapter introduces you to asynchronous messaging using JMS. Understand JMS architecture Understand various JMS messaging models
Environment Setup
1.X.jar Run the env. C:\JavaTraining\j2eelib\j2ee. http://incubator. set CLASSPATH=.2. Copy the activemq-3.
Chapter Goals
Understand the notion of messaging systems.2.org/activemq/activemq-32-release.apache.bat with the following contents. This completes the environment setup.jar.3-bin.jar file from C:\ActiveMQ\lib directory into j2eelib directory.
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.bat file before running the examples. By the end of this chapter you’ll know the basics of messaging systems and how synchronous and asynchronous messaging is implemented.html 2.C:\JavaTraining\j2eelib\ activemq-3. Create C:\JavaTraining\chapter17\env.C:\JavaTraining\j2eelib\mysql -connector-java-5. Download the ActiveMQ messaging system at the following URL.0..

This means the sender can send messages at any time. So. JMS stands for Java Messaging Service.
Importance of Asynchronous messaging
One of the primary benefits of asynchronous messages is that the sender and receiver of the messages can be time independent. Let’s say it has to send orders to three vendors namely A. right? Yes it will be lost. This is a two-way communication. If the inventory system uses synchronous messaging to send orders one by one. Let’s consider a typical eCommerce application which sells wide variety of products from different manufacturers. One of the main components of such application is the inventory management system. the inventory system can temporarily ignore vendor A. Before we understand what JMS is all about.Chapter 17
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Introduction
Introduction
This chapter introduces you to yet another powerful J2EE technology called JMS. This system is required to update the inventory of products based on sales and availability and this has to be in real time. Besides synchronous messaging. Typical examples include email systems. let’s first understand the importance of asynchronous communication. This is where the challenge comes. The simplest example of synchronous communication is HTTP communication in which we send a HTTP request from the browser and the server instantly responds with a HTTP response message. B and C. and the recipient can read the messages at some other time. In this type of communication. What if the recipient of the message is inactive at the time when the sender sends the message? The message is lost in the air. we need to understand the basics of enterprise level communication. which is based on request – response paradigm. right? Why should B and C vendors suffer due to vendor A being down? One solution is. there will be several cases where the communication is required to be asynchronous in nature. In a typical enterprise application most of the communication is synchronous. In this case both the parties namely sender and receiver are always active. you might wonder why enterprise applications really need to do asynchronous communication when it is unreliable. sender sends a request message and receives a response message instantly from the recipient of the message without having to wait for an indeterminate amount of time. Before we discuss a solution for this. message boards. the orders to B and C will also be delayed. then tell me what happens if vendor A is down? Should the system wait until A comes up? If this the case. Most of the enterprise communication falls in this category. The sender doesn’t even care whether the recipient received the message or not. This is one way communication in which sender sends a message to the recipient and doesn’t wait for any response or confirmation from the recipient.
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. discussion forms etc.

2. all it has to do is send the appropriate messages to the middle ware. receipient notification and transaction control.
MoM (Message oriented Middleware)
Following are the key features of any MoM system: 1. This will totally eliminate the dependency of inventory system on vendors A. This is referred to as quality of service.
Vendor A
Inventory
Middle Ware
Vendor B
Vendor C
Fig 17.Chapter 17
Java Messaging Service (JMS)
MoM (Message oriented Middleware)
process B and C and come back to A at some other time. Supports reliable and fast communication 3. This is an additional burden on the system. B and C). B and C. Similarly. The presence of MoM in between the parties is the solution. Guaranteed message delivery. All we did is broke the synchronous communication into asynchronous communication by introducing the middleware. The MoM will be built in such a way that it 100% guarantees the delivery of the messages to recipients (vendors A . Following figure demonstrates this form of messaging. that’s it. Remember I said at the beginning about a solution to reliable asynchronous messaging? Here it is. From inventory system point of view.1 Typical Aysnchronous Messaging
The middleware is now the heart of the application. This is not a good approach because the inventory system should now keep a track of which orders are placed and which are not.
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. Infrastructure that asynchronously connects multiple systems. It is vendor’s responsibility to read messages from the middle ware and update the inventory. The communication between inventory system and the middleware is 1-way which is asynchronous messaging. the communication between vendors and middleware is also asynchronous. right? The simple solution is to have the inventory system send all the orders to an “always running middle ware system” and then have the three vendors read the orders from the middle ware system. This middleware is what we call as MoM system (Message oriented Middleware).

2 A typical MoM based application
Let me ask you a question here. Now that we know some basics about messaging systems. Following picture shows a typical MoM with producers and consumers of messages.
Messaging Models
There are two popular messaging models that every MoM system supports. Point-to-Point model
Publish/Subscribe Model
As the name suggests. MoM systems also support synchronous communication. The consumers of the messages will subscribe with the published topics
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. This is an open source messaging broker. the producer publishes the messages to something called Topics. 5. our goal is to send and receive messages to/from the MoM system. in this model. Hides all the underlying communication details from producers and consumers of messages. Please see the environment page for installing this product. let’s see the different messaging models these systems support. In this chapter. This is a commercial product. There are several free and commercial MoM systems available in the market. but is available as 60 day trial version. Publish/Subscribe model 2.
Consumer A
Producer A Middle Ware
Producer B
Consumer B
Producer C
Consumer C
Fig 17. The popular one is the MQSeries from IBM. I am going to use ActiveMQ product from Apache as the MoM system. MoM can be viewed as a database of messages. Does MoM only support asynchronous communication (1-way)? No. As an application developer.Chapter 17
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Messaging Models
4. These are: 1. Eliminates the need for all systems to be available simultaneously.

Queue is an administered object. In this model. Fig 17. Fig 17.3 shows a publish subscribe messaging model. The examples of this model are newsletters where we subscribe with topics called “Sports” to receive all the sports related news.Chapter 17
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Messaging Models
and receive the messages.
Publisher 1
Sports
Subscriber 1
Publisher 2
Travel
Subscriber 2
Fig 17. This is a one-to-one messaging model. Unlike in pub/sub model. This is one-to-many messaging model in which multiple consumers can subscribe to the same topic and receive the messages. and the consumer reads the message from the queue. Topic is an administered object. Once the message is consumed by the consumer it will no longer be available in the queue.4 Point-to-Point messaging model
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.
Point-to-Point Model
In this messaging model.
Producer 1
Queue 1
Consumer 1
Producer 2
Queue 2
Consumer 2
Fig 17. a producer puts a message into something called a Queue. only one consumer can receive the message that is put in the queue.3 Publish-Subscribe model
As you can see from the above figure. In this model. A single consumer can also subscribe with several topics. one publisher publishes the messages to a topic named sports and both the subscribers subscribed to this topic to receive messages.4 shows this model.

JMS API
JMS API is a set of simple classes we use to send and receive messages to and from MoM systems. Using the admin objects. no matter what the messaging model is. So. we will see example using both the messaging models. In this chapter. 1. there will also be some applications that use pub/sub model. 3. Fair enough? Let me ask you a question here.5 in the next page outlines the process.Chapter 17
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JMS API
Most of the real world applications use point-to-point model due to the flexibility it offers. An Admin tool will bind the administered objects (Topics. Java application will then use JNDI to lookup the admin objects. However. Since MoM is a proprietary messaging system that can be implemented in any language. Good. we will use ActiveMQ MoM broker from Apache. This technology is nothing but JMS.Queues etc) in the JNDI namespace. to build synchronous/asynchronous applications in Java using MoM systems. As you can see from the figure. how do we access it from Java? The answer is simple. there are three steps we usually do to work with messaging system. 2. Similarly we also have a J2EE technology that provides a universal way of accessing any MoM system. There is a standard process we follow while working with MoM systems. In this chapter. Fig 17. the application establishes a connection with the messaging system and performs the messaging. which stands for Java Messaging Service.
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. Recall how we used JDBC to access any type of database. we need to use JMS API.

Just think of it as a Java based solution for storing and retrieving the objects from a common storage place (a.Chapter 17
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1 Admin Tool
JNDI NameSpace
2
Java Application
3
MoM (ActiveMQ)
Fig 17.
JMS Administered Objects
Administered objects are nothing but pre-configured objects that encapsulate all the information about the underlying messaging system. There are only four administered objects in JMS. These are: For Pub/Sub model TopicConnectionFactory Topic For Point-to-Point model QueueConnectionFactory Queue
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.a registry) for Java objects. port number etc. These objects are usually configured by the application administrators.5 JMS and Messaging Systems
For now. The information is usually the hostname. used for connecting with the system. When we write programs. As a developer. you’ll see how we can use JNDI.k. we just need to use the admin objects and connect with messaging systems. don’t worry about the details of what JNDI is.

Create a Topic/QueueConnection from the factory class Start the connection Create a Topic/QueueSession from the connection Use JNDI to search and retrieve Topic/Queue Create either TopicPublisher/QueueSender from session Publish/Send the messages Subscribe/Receive messages Close the connection
For point-to-point messaging we use Queue related classes and for pub/sub messaging we use Topic related classes. and classes that support point-to-point messaging start with the word Queue.
JNDI lookup for Administered objects
All the admin objects namely TopicConnectionFactory and Topic for Pub/Sub model and QueueConnectionFactory and Queue for point-to-point model are bound
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. 4. JMS API has classes for both Pub/Sub model and Point-to-Point model. 7. The class names are very easy to remember as you can see from the following table.1 JMS API Pub/Sub API TopicConnectionFactory Topic TopicConnection TopicSession TopicPublisher TopicSubscriber Point-to-Point API QueueConnectionFactory Queue QueueConnection QueueSession QueueSender QueueReciever
Following is the standard process to work with JMS. 5.
Using JMS API
The usage of JMS API is pretty simple. There are few important classes we need to know. 2. and we use them with a simple and standard process. 1.Chapter 17
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JMS API
We will see what these are and how to use them in later sections. 3. 9. Use JNDI to search and retrieve Topic/QueueConnectionFactory objects. 8. Table 17. All the classes in the Pub/Sub model start with the word Topic. 6.

Publish/Subscribe Messaging using JMS
For this messaging model. ActiveMQInitialContextFactory").PROVIDER_URL."tcp://localhost:61616"). QueueConnectionFactory queueFactory = (QueueConnectionFactory)context.createTopicConnection().INITIAL_CONTEXT_FACTORY.
TopicConnectionFactory topicFactory = (TopicConnectionFactory)context. These two properties will change based on the underlying messaging system.
Creating a TopicSession (Step 4)
The JMS method signature to create a topic session is:
public TopicSession createTopicSession(boolean transacted. The above factory class is an ActiveMQ proprietary class."org. For ActiveMQ. we use the TopicConnectionFactory object obtained from the above look up.apache. As an application developer. int acknowledge)
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.lookup(“ConnectionFactory”). The first thing we need to do before looking up for admin objects is to obtain a JNDI context by supplying in the MoM information. props. and the URL with the hostname and port number as shown below:
Properties props = new Properties(). Context context = new InitialContext(props).jndi.setProperty(Context. This information is usually the context factory class of the messaging system.
Obtaining the connection and starting it (Steps 2 & 3)
TopicConnection topicConn = topicFactory. Once the context is obtained.setProperty(Context.
The factory class is vendor specific just like a driver class for database. we need to use JNDI API and look up (search) for these objects. props. these objects are bound with the names “ConnectionFactory”.start(). The lookup for objects will therefore be as shown below. we use other topic related classes in JMS API to publish and subscribe messages. topicConn.Chapter 17
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to the JNDI namespace in the server. Using this factory object. we do a lookup (search) for administered objects namely.activemq. QueueConnectionFactory or TopicConnection Factory.lookup(“ConnectionFactory”).

Chapter 17
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Publish/Subscribe Messaging using JMS
Method Arguments: transacted – If true. we create the session as shown below:
TopicSession topicSession = topicConn. the session will be transacted. 3. CLIENT_ACKNOWELDGE – The recipient will explicitly acknowledge the message by calling the acknowledge() method Using the above method.
Steps 1 though 5 are common for both Publisher and Subscriber code. 4. Following are some of the important JMS messages: 1.
Creating a Publisher
Create a publisher for the topic using the session as shown below (Step 6)
TopicPublisher publisher = topicSession.createPublisher(topic).createTopicSession(false. AUTO_ACKNOWELDGE). TextMessage (Widely used for sending text messages) ByteMessage (Used for sending bytes of data) ObjectMessage (Used for sending objects) MapMessage (Used for send data as name-value pairs)
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. You can copy paste them. Following two steps are used in Publisher code. acknowledge – An integer that specifies the acknowledgement mode by the recipient of the message.
Obtain the Topic from JNDI (Step 5)
Topic topic = (Topic) context.Session.lookUp(“dynamicTopics/Sports”). The usual values are AUTO_ACKNOWELDGE – The message will be automatically acknowledged by the recipient session.
Creating Messages
Create and publish messages using the above publisher object (Step 7) There are different types of messages we can create based on the type of data to be sent to the recipient. 2.

To publish the above message to the topic.createTopicSession(false.
The overall consolidated code for the publisher will be as shown below:
public void sendMessage() throws JMSException {
TopicConnection topicConn = topicFactory.Session. Durable subscriptions Let’s see what these are. The subscriber code should register a listener object that should continuously listen and read the published messages. TopicSession topicSession = topicConn. publisher. we use the TextMessage class and specify the text as shown below:
TextMessage message = topicSession. Non-Durable subscriptions 2. This type of subscription will consume the messages as and when they are published.
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. Topic topic = (Topic) context. }
Coming to the subscriber part. message. there are two types of subscriptions we can make as shown below: 1.
Non-Durable subscriptions
With non-durable subscriptions.Chapter 17
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To send a text message.setText(“A simple message”). message. TopicPublisher publisher = topicSession. topicConn.createTopicConnection(). the subscriber application should be up and running and continuously listen for messages. AUTO_ACKNOWELDGE).publish(message).lookUp(“dynamicTopics/Sports”).e.publish(message).start(). TextMessage message = topicSession.createPublisher(topic).setText(“A simple message”). we invoke the publish() method as shown below:
publisher.createTextMessage(). the subscriber of the topic will receive the published messages only when the subscriber connection is active.createTextMessage(). i.

This completes the subscriber code.setClientID(“Bond”). If any messages are published during this inactive time.getText().
Create and set the Listener
To create a listener object. we need to write a class that implements MessageListener interface and implement the onMessage() method as shown below:
class MyListener implements MessageListener{ public void onMessage(Message msg){ // Cast to what ever message published TextMessage tm = (TextMessage)msg.Chapter 17
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In order for the subscriber not to loose any of the published messages. Following are the steps for creating nondurable subscriptions
Creating a Subscriber
The subscriber for a particular topic should be created as shown below:
TopicSubscriber subscriber = session. With durable subscriptions. it should be registered with the subscriber as shown below:
subscriber. System. It is now ready to listen for published messages.
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. the messaging system will persist them internally. so that when the subscriber comes alive.createSubscriber(topic). there is one important thing called clientID that should be set on subscriber side.
Durable Subscriptions
With durable subscriptions.setListener( new MyListener() ). the subscriber of a topic can receive messages even if the subscriber connection is inactive. The code for durable subscriber will look as shown below: On Subscriber side:
conn. he will receive the earlier messages and also the messages from that point onwards.out. the subscriber code must be run before the publisher code. } }
Once the listener class is created.println(message). String message = tm.

Now. How are you doing
Now.Chapter 17
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Publish/Subscribe Messaging using JMS
Now. This is a proof of one-many messaging in which any number of subscribers can subscribe to the same topic and every subscriber will receive the published messages. This is because the messaging system retains the messages internally so that when the subscriber comes active. you’ll see the messages twice. }
Save the code and run the Subscriber and Publisher programs again. How are you doing Hi Smith. The subscriber must be executed before the publisher. you’ll also see the actual three text messages in the Subscriber window as shown below:
Hi John.createTopicConnection(). This completes the Pub/Sub messaging model. The subscriber will receive the messages. Save the file and run the Publisher first and then the Subscriber.
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. Let’s see how Point-to-Point model works."Bond"). open another command window and run the Publisher code using the following command. let’s modify the subscriber code to use durable subscription. This is an example of non-durable subscriptions. and you’ll never see the published messages. First you need to change is set the clientID on the connection object as shown below:
TopicConnection conn = tcf. conn.Publisher Messages Published
The moment you see the above message.setClientID(“Bond”).
That’s it.
C:/>JavaTraining>chapter17>java jms. How are you doing Hi James.
We then have to create a durable subscriber instead of regular subscriber as shown below:
TopicSubscriber subscriber = session. This is because the subscriber is inactive when the messages are published. change the main method to add one more subscriber as shown below
public static void main(String[] args) { new Subscriber(). Try running the publisher code first and then the subscriber code. This time in the Subscriber window. new Subscriber().createDurableSubscriber(topic. it will deliver the messages.

start().lookUp(“dynamicQueues/Sports”). AUTO_ACKNOWELDGE).
Obtaining the Queue factory
The factory class for this model is obtained from JNDI lookup as shown below:
QueueConnectionFactory queueFactory = (QueueConnectionFactory)context. acknowledge – An integer that specifies the acknowledgement mode by the recipient of the message.
Obtain the Queue from JNDI (Step 5)
Queue queue = (Queue) context.Session.createQueueConnection(). the session will be transacted.createQueueSession(false.
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. The administered objects in this model are QueueConnectionFactory and Queue.lookup(“ConnectionFactory”). int acknowledge)
Method Arguments: transacted – If true. queueFactory.
Creating a QueueSession (Step 4)
The JMS method signature to create a queue session is:
public QueueSession createQueueSession(boolean transacted. CLIENT_ACKNOWELDGE – The recipient will explicitly acknowledge the message by calling the acknowledge() method Using the above method.Chapter 17
Java Messaging Service (JMS)
Point-to-Point Messaging using JMS
Point-to-Point Messaging using JMS
The JMS API for this model looks much the same as Pub/Sub model except that the class names will begin with the word Queue instead of Topic. The usual values are AUTO_ACKNOWELDGE – The message will be automatically acknowledged by the recipient session.
Obtaining the connection and starting it (Steps 2 & 3)
QueueConnection queueConn = queueFactory. we create the session as shown below:
QueueSession queueSession = queueConn.

createQueueConnection().send(message).lookUp("dynamicQueues/Sports"). message.
To send the message to the consumer.start(). TextMessage message = QueueSession. AUTO_ACKNOWELDGE). QueueSession queueSession = QueueConn.setText(“A simple message”). we invoke the send() method as shown below:
sender. You can copy paste them.
The overall consolidated code for the producer will be as shown below:
public void sendMessage() throws JMSException {
QueueConnection queueConn = QueueFactory.createTextMessage().
Sending Messages
Create and send messages using the above queue sender as shown below (Step 7)
TextMessage message = topicSession.createSender(queue).
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.createTextMessage(). sender. message.send(message). Queue queue = (Queue) context.createQueueSession(false. Following two steps is used in Producer code. }
With point-to-point messaging. there will be only one consumer for the messages unlike in pub/sub model where we have multiple subscribers for messages. the message will be persisted in the queue until someone consumes the message. When the producer send message to the queue.Session. QueueSender sender = QueueSession. queueConn.createSender(queue). The messaging system ensures that the message is never lost.
Creating a Sender
Create a sender for the queue using the session as shown below (step 6)
QueueSender sender = queueSession.setText("A simple message").Chapter 17
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Point-to-Point Messaging using JMS
Steps 1 though 5 are common for both producer and consumer code.

println("Reading message: " + msg.lookup("dynamicQueues/Movies").out.setMessageListener(new MyListener()). } catch (Exception e) { e.CLIENT_ACKNOWLEDGE). } } }
If you look at the above code. conn. QueueReceiver reciever = session. System.createReceiver(queue). Session. This is a one-toone messaging model.out. The consumer can run at any time and receive the messages. the messages are always persisted in the Queue when the producer sends the messages.Chapter 17
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QueueSession session = conn.getClass().getName()). Since this is point-to-point messaging. } } class MyListener implements MessageListener { public void onMessage(Message message) { TextMessage msg = null. Queue queue = (Queue) context.createQueueSession(false. reciever.close(). Once the message is consumed. try { if (message instanceof TextMessage) { msg = (TextMessage) message.printStackTrace().
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.println("Message of wrong type: " + message. } else { System. You cannot have multiple consumers consume the same message from the same queue. Compile and execute the Producer code using the following commands.printStackTrace(). } } catch (Exception e) { e. the messaging system will permanently delete the message. } } public static void main(String[] args) { new Consumer().getText()). the producer sends a message to a queue named “Movies” and the consumer reads the message from the same queue. The only difference in the two programs is that one uses QueueSender object to send the message and the other use the QueueReciever object to receive the message.

Synchronous messaging is a 2-way communication in which every request will be responded immediately.Chapter 17
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Summary
C:/>JavaTraining>chapter17>javac jms\*. Just try to understand how the Pub/Sub and Point-to-Point messaging works and you’ll be fine.java C:/>JavaTraining>chapter17>java jms.
C:/>JavaTraining>chapter17>java jms. a single queue can only have one consumer. I am sure you now understood how asynchronous messaging works. Publish/Subscribe and Point-to-Point are the two messaging models that every MoM system supports. Asynchronous messaging is 1-way communication in which sender sends the message and doesn’t wait for any response from the recipient. With durable subscriptions.Producer
You’ll see the message “Message Sent” in the console. Subscriptions can be durable or non-durable in nature.
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. publisher will publish messages to several topics.
Hello John.
Summary
Enterprise applications typically use Message Oriented Middleware (MoM) for both synchronous and asynchronous messaging with the backend systems. Most of the real world applications use one of two messaging models. the recipient can receive messages even while being inactive. This completes all the important concepts of messaging. Now run the consumer code using the following command. Subscribers subscribe with the topics to receive messages. This is a one-to-many messaging model. If you need to send messages to several consumers. JMS technology provides universal way for interacting with messaging systems. With non-durable subscriptions the subscriber application must be up and running to receive the messages. In pub/sub model.Consumer
The consumer code will display the following result in the console. MoM system acts a middleware between senders and recipients. How are you doing?
In this messaging model. you need to create a separate queue for each and every consumer.

Chapter 17
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Time to play 50-50
In point-to-point model. Which of the following is an example for MoM system? a) b) MySQL MQSeries
2. This message will them be consumed by the consumer. This is a one-to-one messaging model.
Time to play 50-50
1. sender send a message to queue. Which messaging model uses queues for sending and receiving messages? a) b) Pub/Sub Point-to-point
Interview Questions
Question: What are the different messaging models supported by JMS? Answer: Publish/Subscribe and Point-to-Point models
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. Which of the following is an administered object? a) b) ConnectionFactory TopicConnectionFactory
5. Which of the following is a 1-way communication? a) b) Asynchronous Synchronous
4. The MoM system retains the message in the queue until it is consumed. Which J2EE technology is used for accesing MoM systems? a) b) JNDI JMS
3. Which of the following messaging model uses topics for messages? a) b) Point-to-Point model Pub/Sub model
6.

A single publisher can publish messages to more than one topic and a single subscriber can subscribe with any number to topics. Question: How does point-to-point messaging works? Answer: Point-to-point messaging is a one-to-one messaging in which one party sends a message to the queue and the other party reads the message from the queue. the message broker will persist the message until it is consumed by some party. In this model.
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Interview Questions
Question: Explain how Pub/Sub model works? Answer: This is a one to may messaging model in which a publisher publishes messages to topics and subscribers will register with one or more topics to receive messages.

jar.bat file with the above Jar files just like we did in the previous chapters. commons-beanutils. Set the CLASSPATH in the env.org/download 2.jar.jar.
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.jar
3. commons-logging. make sure you download the distribution with dependencies. http://www. commons-collections.
Chapter Goals
Understand Spring architecture Inversion of Control Aspect Oriented Programming Understanding spring core module Understanding DAO module. After clicking the download button.springframework.
spring-1. Copy the following Jar files in the Spring installation directory to the “j2eelib” directory.2. commons-dbcp.2.jar. This chapter explains the important spring modules. Download the Spring framework at the following URL.Chapter 18
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Chapter Goals
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This chapter introduces you to yet another open source framework for building enterprise applications.
Environment Setup
1. the Spring framework.

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. This is where we start wondering if there is any alternative solution/technology that completely eliminates the EJB container. Isn’t this what we are looking for? EJB containers usually come along with application servers which are mostly commercial and are expensive.test etc. persistent services etc.Chapter 18
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Introduction
This chapter introduces you to one of the most popular and widely used open source framework. Though EJB technology offers several benefits. who can stop anyone from choosing spring framework to build enterprise applications? This is why we are interested in knowing something about spring framework and the features it has to offer to the developer community. Some enterprise applications use EJB technology to build business and persistence components. test.test. The question that comes to our mind at this point is. enterprise applications to leverage these features from EJB container use EJB components to build business and persistent components. the Spring framework. Spring framework not only eliminates the necessity of EJB container but also comes at absolutely free of cost. persistence services. So. The testing cycle will therefore be code. One such technology is the EJB technology. Spring is a light weight container framework for building Java applications inside and outside the container.code. Spring is a lightweight framework that provides most of the important services like transactions. code. there are some technologies in it that consume whole lot of time in terms of development and testing. Instead they only require using one or two of the important container services such as transactional. The one and only one reason for choosing EJB is the support that EJB container provides to the EJB components such as transactional. One of the key challenges today’s J2EE developers face is to build applications faster and that are easy to maintain with minimum effort. persistence that can be used out of the box. start server. The EJB container is like a server that needs to be restarted for every change in the business component. testing time and probably even performance. Although the standard J2EE technologies offer solutions for rapid application development. This is where spring framework steps in. yet provides most of the frequently needed services to the business components. Most of the real world applications are not complex enough to really use the full scale services of the EJB container.test etc which should ideally be code. start server. So. the usage of EJB container to host the EJB’s causes additional overhead in terms of consuming resources and most importantly effecting the length of testing cycle. why the heck do we need to use this heavy weight EJB container just to leverage few services and more so at the expense of resources.

Instead. As your business starts growing and the business requirements are getting complex and complex that are tough to handle by spring alone. With spring framework. we can build components that can be used across all the tiers in a typical n-tier application. In this chapter. Until then let’s use spring framework. You can imagine this framework as another Java alternative solution for J2EE.Chapter 18
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Let me tell you one thing here. Keep the life of J2EE developer simple Spring doesn’t use any complex business components like EJB’s. Though we demonstrated our hatred towards EJB. If you are starting a business. it can be used for building web components. then you can think of EJB container. interoperability etc which are beyond the reach of the most of the real world enterprise applications. What does this mean? If we take Struts for instance. 2.
Goals of Spring Framework
1.
Spring Framework
Spring framework unlike other popular open source frameworks like Struts is not a specialized framework. right? This is when you can use spring framework. In presentation tier. you want to build the infrastructure at minimum cost. in the business tier it can be used for building transactional and persistent components etc. security. it uses the simple Java Beans which we also call as POJO’s to build applications by making them almost as powerful as EJB’s. we cannot completely rule out the usage of EJB container just because of the above few reasons. we’ll look at the basic architecture of spring framework and also utilize its cool development tools to build better applications are that are easy to maintain and easy to test. EJB technology is built to meet the highly complex business requirements such as distributed transactions. Developing POJO’s hardly takes few minutes and also simplifies the testing of business components. remoting. This allows testing the components individually
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. it can only be used for building web application components that only reside in the presentation tier. Improve the loose coupling of application components When developing business components it’s very important that they must be as loosely coupled as possibly can. It’s more of a generalized framework for building end-to-end J2EE applications.

the entire spring framework is divided into several modules as listed below:
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. We’ll see IoC in detail in later sections.
AOP Module
DAO Module
ORM Module
WEB Module
Application Context Module
Core Module
Fig 18. Knowing all the above good things about spring framework. Reduce the application maintenance burden With the applications no longer using heavy weight components like EJB. Spring uses the notion of Inversion of Control to achieve loose coupling. let’s begin our journey into the world of spring framework. 3. code maintenance gets very easy from the point of packaging the code to deploying the code on the server. It also reduces the complexity of configuring the application components.1 Architecture of Spring framework
As you can see from the above figure.
Architecture of Spring framework
Following figure shows the high level view of the spring framework.Chapter 18
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without any dependencies. This has a direct effect in improving the performance of the application. 4. Improve the performance by consuming fewer resources Being a lightweight framework. spring doesn’t consume system resources heavily for providing services to application components.

Application Context Module
This module is an extension to the core module in the sense it offers services such as access to JNDI components. It includes all the core infrastructure components that support the framework and also factory classes for bean processing. 3. This way it
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.
Core Module
The core module is the heart of spring framework. 5. One such factory class is the BeanFactory class which is used for working with POJO’s or simply JavaBeans. bean properties can be loaded from property files. This factory class allows wiring the beans on the fly and also used to populate the bean properties. This module offers highly simplified solution for persisting data to databases. Using AOP we can cleanly eliminate duplicate code (ex. The reason is simple. security logic or logging logic) that is usually scattered across various business methods. Using this module. Though spring has several other modules.
Core Module Application Context Module AOP Module DAO Module ORM Module and Web Module
Let’s see the purpose of each individual module. 2..
AOP Module
This module provides support for Aspect Oriented programming. most of the real world applications uses spring framework just for leveraging the services of this module. 6. This module abstracts all the redundant JDBC code like loading the drivers. It does this though something called templates which we’ll see later. AOP capabilities are not yet completely utilized in the world of J2EE.Chapter 18
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1. but it is very powerful feature that declaratively injects utility services to business components. and more importantly support for internationalization through the use of property files and resource bundles. 4. managing the connections etc and allows users to only work with the important things like executing the SQL queries and processing the results.
DAO Module
The DAO module is what made spring very popular.

Inversion of Control. Don’t be surprised if you only get DAO module related questions in interviews. This also means that all the time we spent learning struts didn’t go in vain. AccountServiceImpl and CustomerService Impl. This chapter only covers the most important and frequently used modules listed below: 1.
Inversion of Control (IoC)
Inversion of Control is not a technology or anything even close. To better understand IoC. but can integrate third party ORM solutions like Hibernate. let’s understand the first concept. Let’s say we have a bank teller whose purpose is to provide two types of services to customers namely account services and customer services.
ORM Module
This module is used for working with Object Relational Mapping tools. we will also learn the concepts of Inversion of Control and Aspect Oriented Programming. Spring doesn’t come with a full implementation of ORM solution. Once integrated. let’s first look at an object design without IoC and see how it impacts the testing of application components. Now that we know spring architecture and the modules it has. It’s just a kind of design practice that promotes loose coupling of application components thereby improving the quality of testing the code.
Web Module
This module as the name suggests can be used for building web applications. This module like struts framework is built upon MVC design pattern. Consider the following object design for the three objects Teller. The good and bad news is that this module didn’t gain as much attention as DAO module. Core and Application Module 2.Chapter 18
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keeps the persistence code lot simpler and cleaner. To cover all the above modules is beyond the scope of this book. Most real world applications even today still use Struts framework for building web applications. this module provides a consistent way of persisting data by hiding the underlying ORM solution.
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. DAO Module Besides the above modules. Let me tell you one thing here.. JDO etc.

transferFunds(). it throws an exception with the message “Unable to Transfer Funds”. public class TellerTest { public static void main(String args[]) throws Exception { Teller t = new Teller(). This is therefore a bad test. public class Teller { private CustomerServiceImpl cImpl = new CustomerServiceImpl(). we can conclude that the teller class as a failure since the exception occurred while invoking the teller class. The Teller class references the above two classes to invoke the appropriate methods. Can you think
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package spring. This is possible only if we eliminate the dependencies. let’s write the test class for Teller. Therefore.
package spring. With all these three classes. private AccountServiceImpl aImpl = new AccountServiceImpl(). A test class should only test the functionality of the corresponding class and not its dependent classes. the AccountServiceImpl class defines a single method namely transferFunds(). } }
As you can see from the above code. This method throws an exception if the account is not found. If we execute the test class. But if you carefully look at the code. A good design must allow testing the classes independently irrespective of the dependent classes.changeAddress(). the exception is actually thrown by the transferFunds() method in the AccountServiceImpl class.updateAddess(). } public void updateAddess() { cImpl. } }
The above test class simply invokes both the operations on the teller class. The main reason for this is the tight coupling between Teller class and its dependent classes. So the question is. But from the testing point of view the culprit seemed to be the Teller class. t. Similarly CustomerServiceImpl class defines method to change customer address. t.transferFunds(). does the above code really test the Teller class? The answer is big No. public void transferFunds() throws Exception { aImpl.

} }
The code for AccountService will also follow the same lines. } }
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of a solution for this? Very simple. Let’s see how we can use interfaces and eliminate the dependencies. public void transferFunds() { aService.aService = aService. } public class CustomerServiceImpl implements CustomerService{ public void changeAddress(){ System. } public void setCustomerService(CustomerService cService) { this. } public void setAccountService(AccountService aService) { this. With these two updated classes.cService = cService. Let’s have the CustomerServiceImpl class implement the CustomerService interface as shown below:
public interface CustomerService{ public void changeAddress().println("Address has beeen updated").changeAddress(). let’s now look at the Teller code.out. private AccountService aService.transferFunds(). Listing 18.1d (Teller. We simply need to reverse the design by injecting the dependent classes to the Teller class rather than having the teller class reach for its dependencies. } public void updateAddess() { cService. This decoupling is achieved by using the so called interfaces.java) Updated class
public class Teller { private CustomerService cService.

any failure in the test code from can be attributed solely to the Teller class itself. } }
As you can see from the above code. it’s very important that they be associated with each other in some way or the other. B. A typical spring application is comprised of several independent beans which we also call them as POJO’s (Plain Old Java Objects).Chapter 18
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As you can see from the above Teller class code. t. t. Inversion of Control is a design technique that spring uses behind the scenes.setAccountService ( new AccountServiceImpl() ). The class is now completely independent of CustomerServiceImpl and AccountServiceImpl classes. All this is possible only by injecting the dependencies which is what is what we call as inversion of control. t. and then invoke the actual business methods as shown below:
public class TellerTest{ public static void main(String args[]){ Teller t = new Teller(). we introduced two setter methods to inject the implementation classes using the interface references. the dependent objects are passed to the teller class prior to invoking the business methods. If the two impl classes are tested successfully prior to testing Teller class. In simple words.updateAddress(). We call this as wiring the beans. if we have three classes namely A.setCustomerService ( new CustomerServiceImpl() ). in spring we use the notion of beans to implement functionality.C are dependents of A. Therefore the test class should first invoke the setters to send the dependent objects. Just like in object oriented programming where we have independent objects that are associated with each other to implement a solution.transferFunds(). The Teller class can be viewed as a light weight class since it cannot do anything until the actual implementation objects are injected into it. With this basic understanding of IoC. C where B.
Core Module
The spring’s core module essentially comprises of factory classes for working with Java beans. In order to get the functionality of out of these beans. then according to IoC. certain business requirement
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. t. For instance. B and C must be injected in to A rather than A reaching for B and C. let’s start looking at the spring modules one by one.

Let’s see how this is done. This is the most widely used factory class in real world applications. The spring’s core factory classes provide services for wiring the beans.
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.
Why wiring beans?
In typical applications. Although from client’s point of view the only bean that needs to be loaded is the Teller bean. Besides the basic support. the teller should then wire the customer and account beans etc. For instance look at the following bean structure:
Customer Client Teller Account CreditCard
Mortgage
The above figure shows the bank teller functionality. Spring uses two container classes namely BeanFactory and ApplicationContext for managing the beans. The teller application needs to use Customer and Account bean for certain business functions. Before we process them we need to populate the beans properties and associate them with each other.Chapter 18
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may require the services of Account bean and Customer bean. Wiring the beans usually starts from the bottom node and wiring up to the root node. It also allows the bean properties to load from property files. The account bean in turn uses some other beans to fulfill the teller’s requests. it also provides internationalization features though the use of property files and resource bundles. most of the functionality will be collectively implemented by several beans. The ApplicationContext class however is much more sophisticated and is an extension to BeanFactory class. The BeansFactory class is a simple container that provides the basic services like injecting dependencies through Inversion of Control. it’s very important that we also load the behind the scenes beans for the teller bean to work as desired. This means the account bean must first wire the CreditCard and Mortgage beans.

This factory uses the XML file that contains the definition of all the beans for creating and loading the bean properties. This example defines a customer bean and loads the properties firstName. Before we understand the complete intricacies of beans.getBean(“customer”). let’s do a simple example to get a feel. we can start creating beans using the getBean() method. lastName etc with some arbitrary values. Listing 18. The XMLBeanFactory class is instantiated as shown below:
BeanFactory factory = new XMLBeanFactory( new FileInputStream(“MyBeans.java) Simple Java Bean
package spring. Loads the bean properties with the appropriate values.2a (Customer.xml”)).
BeanFactory
The BeanFactory class as the name suggests is a factory of bean classes. Its main purpose is to instantiate and set the properties of different types of bean classes. Instantiates the bean 2.example1. the getBean() method before returning the bean instance. One such implementation class is the XMLBeanFactory which is one of the widely used.
In the above. Every bean in the XML is identified with a unique identifier which is used to retrieve the bean as shown below:
MyBean bean = (MyBean) factory. customer is the id of the bean defined in the XML. Ok. The definition for the bean in the XML file will look as shown below:
<bean name=”customer” class=”beans. does two important things: 1.MyBean”/>
As we all know the fact that a typical bean stores the data in the form of properties whose values are set and retrieved using the setter and getter methods.Chapter 18
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Now that we know the importance of wiring beans.2. let’s see the two important core components namely BeanFactory and ApplicationContext classes that aid in wiring the beans. The BeanFactory is an interface that has multiple implementation classes. Take a look at the code in listing 18.
Once the reference to factory object is obtained. public class Customer {
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.

The bean element does the following: 1. Steps to run the program 1. Compile the two classes as
C:/>JavaTraining>chapter18>javac spring\example1\*.Chapter 18
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The Customer bean doesn’t deserve any explanation. FileSystemXmlApplicationContext: This class is used to load the bean definitions XML file from the file system. There are multiple implementations available to this interface. Save the files as
C:\JavaTraining\chapter18\spring\example1\Customer.
ApplicationContext
As said before.CustomerTest
The result of the above program will be
Bond:99999
Whenever we write a bean class.java
3. however following are two most important ones.
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.xml
2. When the bean is returned by the factory class.java C:\JavaTraining\chapter18\mybeans. The xml file uses the bean element to define the bean. Specifies the id and fully qualified class name of the bean 2. Execute the test class as
C:/>JavaTraining >chapter18>javac spring.example1. Look at the test class. and then retrieved the bean by its id. this container class is also used for working with the beans. spring will load the bean properties with the values in the bean definition.java C:\JavaTraining\chapter18\spring\example1\CustomerTest. Now let’s look at the second factory class for wiring beans. we also need to define the bean in bean definitions xml file. This class loaded the xml bean definitions file. Set the bean properties using the nested property elements. The most significant improvement with this class is that it also loads the properties from the property files.

beans will be retrieved as.
MyBean bean = (MyBean) ctx.
Once the context is obtained.springframework.xml”). we need to define a bean of type PropertyPlaceHolderConfigurer which is a built-in spring class and specify the property file location as shown below:
<bean id="propertyConfigurer" class="org.factory.xml”).
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. You are free to use which ever you want./mybeans.beans.mysql.
Similarly. the driverName property can be read as ${driverName} and so on. The following bean definition defines a DataSource bean whose properties are loaded from the above property file.Driver username=test password=test1
In order that the application context load the above property file. we can read the properties using the ${} syntax. The application context using the FileSystemXmlApplicationContext is typically created as shown below:
ApplicationContext ctx = new FileSystemXmlApplicationContext(“c:/…. Consider the following property file
db.
Now. config. Using this context.properties</value> </property> </bean>
Once the above bean is defined with all the information.Chapter 18
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ClassPathXmlApplicationContext: This class is used to load the xml file from the class path.getBean(“customer”). For instance. the application context using ClassPathXmlApplicationContext is created as shown below:
ApplicationContext ctx = new ClassPathXmlApplicationContext(“mybeans. let’s see the how the application context uses the property files to load the bean properties.PropertyPlaceholderConfigurer"> <property name="location"> <value>db.jdbc.properties
driverName = com. its usage is again same as BeanFactory.

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<bean id=”dataSource” class=”beans. Case 3: Referencing other beans.DataSource” init-method=”init” destroy-method=”close” />
The init-method attribute specifies the initialization method. Similarly. This element defines several child elements that can be used to make the bean very powerful.
<bean id=”customer” class=”beans. It also defines another property named userid whose value is read from the property file. and the destroymethod attribute specifies the method before destruction. Sometimes before invoking the business logic methods on the bean. Following definition defines a bean with a property named driver with value “TestDriver”. we need to invoke certain method for clean up.DataSource> <property name=”driver”> <value>${driverName}</driver> </property> <property name=”username”> <value>${username}</value> </property> </bean>
Let’s now see the some of the basic scenarios for defining the bean element in the XML file. One of the most important aspects of wiring the beans is its ability to reference other beans. Let’s say we want to wire the beans as shown below:
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. Following are the important cases: Case 1: Setting the bean properties. before the bean is destroyed. we require to invoke the initialization method. To set the properties of the bean with we use the nested property element. Spring allows us to declare these methods in the XML file as shown below:
<bean id=”ds” class=”beans.Customer”> <property name=”driver”> <value>TestDriver</value> </property> <property name=”userid”> <value>${username}</value> </property> </bean>
Case 2: Initialization and destruction methods.

Credit Card etc.Account” > <property name=”creditCard”> <value> <ref bean=”cc”/> </value> </property> <property name=”mortgage”> <value> <ref bean=”mort”/> </value> </property> </bean>
As you can see from the above definition.
<bean id=”cc” class=”beans. For instance.Account”> <property name=”accountTypes”> <list> <value>Checkings</value> <value>Savings</value> <value>CreditCard</value> </list> </property> </bean>
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. The creditcard and mortgage properties of account bean now references the CreditCard and Mortgage beans. Following definition shows how to wire the collection type properties. some of the bean properties will be represented as collection of objects.Mortgage”/> <bean id=”account” class=”beans. Case 4: Collection properties. We will first define the CreditCard and Mortgage beans and then wire them to Account bean.Chapter 18
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CreditCard Account
Mortgage
Referencing the beans should always be done backwards from right to left or bottom to top.CreditCard”/> <bean id=”mort” class=”beans.
<bean id=”account” class=”beans. Following bean declaration demonstrates this. The factory classes also support the setting of collection properties. More often than not. we used the ref element to reference other beans. the Account bean can have a list of account types namely Checkings. Savings.

Such services include logging.” Hey. and you’ll understand the entire program. In order to plug-in the service declaratively. This is where AOP comes into the picture and says. AOP as the name suggests revolves around the central entity called Aspect. when we are forced to change the signature of the method.
Aspect Oriented Programming (AOP)
Often in applications there will be certain operations or services that are required to be used by all the business components across all the tiers. Aspects one written can be declaratively plugged into the code and this is exactly what spring does. In such cases we normally end up duplicating the code.>JavaTraining>chapter18>javac spring\example2\*.Chapter 18
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c:\JavaTraining\chapter18\spring\example2 c:\JavaTraining\chapter18\mybeans. AOP programming. This approach solves the problem to some extent. right? This again becomes a maintenance burden. look at the following figure:
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. we need to go ahead and update all the references.>chapter18>javac spring. yet leveraging the services”.example2. but not completely in the sense that you’d still end up with duplicate method calls. One simple solution for this problem is to encapsulate all the redundant code in utility methods and invoke the method wherever needed. spring uses the application context. An aspect is nothing but a reusable service that can be plugged in declaratively. To better understand. Execute the test class as
C:/>…. you can use AOP features and completely eliminate even the duplicate method calls. This is very much like a tight-coupling issue. For instance. Let’s see how it achieves this.TestClient
Go through bean definitions. authentication etc. At the same time we cannot leverage the services without invoking them.java
3.xml
2. The test class will produce the following result
Test Driving Honda with Manual trasnmission whose HP is 200 Test Driving Acura with Automatic trasnmission whose HP is 250
This completes all the basics you need to know about wiring. Compile the classes as
C:/>…. Let’s now look at another important concept of spring framework.

So. let’s see how this works and what is needed to be done from development point of view. Based on this information. without a single line of code change you can get rid of it. This contains the service code that is executed at runtime. spring will make the services intercept the business method calls and execute the service code. authentication and database services are independent services that can be used be any application module. This is like an interface to a class.
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Logging Service Account Module Authentication Service
Customer Module Database Service
Fig 18. This is the true beauty of AOP with spring. Tomorrow if you think you don’t want to use a particular service. let’s understand some of its terminology. Before we learn something about AOP. For instance. without wasting any further time. at runtime. the account module uses all three services while the customer module uses just two services.
Aspect
An aspect is the definition of the service we want to implement.2 Business modules and services
As you can see from the above figure. There is nothing wrong here because every module is free to choose whatever services it needed. right? With Spring AOP features. logging.
Advice
An advice is the actual implementation of aspect. business modules can use the application context to specify declaratively what services it needs.

Types of Advices
In spring. or only to some methods of the class? There is no use in writing an advice if we don’t define a pointcut as to where the advice should be applied. Advices of this type are always invoked before the execution of the business method.
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. The spring component that supports this advice is the MethodBeforeAdvice interface. Object target)
Method Arguments method – contains all the information about the target method params – The parameters values of the target method target – The target object itself To better understand this type of advice. should the advice be applied to all the business methods of a class. Object[] params. One thing to be noted with this advice is that the target method is always executed. This interface defines the following method that needs to be implemented:
void before(Method method.Chapter 18
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PointCut
A pointcut defines where the advices should be applied in the code. For instance. there are four types of advices namely. let’s write a logging advice. Once the execution of the advice (service) is completed spring will execute the actual business method. 3. 2. 1. Before Advice After Advice Around Advice Throws Advise
Before Advice
I am sure you already guessed what this advice might do. Now that we know what an advice is and what a pointcut is. 4. let’s learn about advices in detail.

public class LoggingAdvice implements MethodBeforeAdvice { public void before(Method method. we implemented the before() method in the advice. } }
As you can see from the above code.out. import org.java) A simple logging advice
package spring.java) A simple after logging advice
package spring.aop.getClass().AfterReturningAdvice. Object args[]. Object target) throws Exception { System.reflect. Look at the following code: Listing 18.aop.example3. public class LoggingAfterAdvice implements AfterReturningAdvice { public void afterReturning(Object retVal. We will use this code later. Object target) throws Exception {
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. import org. This method simply logs the name of the method along with the class name that the method belongs to.springframework.getName() + "in " + target.reflect.
void afterReturning( Object returnVal.Method.Chapter 18
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Listing 18. An advice of this type will be executed after the completion of the target method.lang.lang. Method method. We can write another logging advice that also logs the return value. Method method.example3.getName()).Method. Object args[]. This interface defines the following method that needs to be implemented.
After Advice
This is the reverse of the before advice.MethodBeforeAdvice. For now. Object[] params. The spring component that supports this advice is AfterReturningAdvice. let’s learn the next advice.4b (LoggingAfterAdvice. import java.springframework. You can even write JDBC logic in this method and log it to database.println("Entered into " + method. Object target) throws Throwable
This method takes one additional argument which is the return value of the method. import java.4a (LoggingAdvice.

MethodInvocation. The spring component that supports this advice is MethodInterceptor. Let’s say we pass a username and password as method parameters that should be verified before the method is executed. The unique feature of this advice is that it allows the user to take the decision whether or not to execute the target method.4c (AuthenticationAdvice. } else { throw new Exception("Invalid Credentials"). you are always guaranteed the execution of target method unless an exception is thrown.aopalliance.getArguments()[0].intercept. String password = (String) invocation. Take the decision whether or not to execute the target method. This is the most popular advice used in real world applications.equals(password)) { ret = invocation. if ("bond".
void invoke(MethodInvocation invocation) throws Throwable
The best example for this advice is the authentication. } }
Around Advice
This advice is can be viewed as a combination of both the before and after advices.println("Exiting method " + method. public class AuthenticationAdvice implements MethodInterceptor { public Object invoke(MethodInvocation invocation) throws Throwable { Object ret = null.proceed().example3. This interface defines the following method that needs to be implemented.aopalliance. }
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.out.MethodInterceptor. import org. Change the return value of the target method. Following is how we implement this scenario using around advice. However with this advice.getName() + "with a return value " + retVal).getArguments()[1].java) A simple around advice
package spring.intercept.Chapter 18
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System. String userName = (String) invocation. and also 2. If you take the before advice. Listing 18. there two things we can do: 1. import org.equals(userName) && "test".

java) A simple exception advice
package spring. With these advices we don’t have the ability to perform some cleanup operations in the event of exception like closing connections to database and all that good stuff. In our case. Throwable exception) { System.lang.ThrowsAdvice. which method do we need to implement? Don’t worry. It allows us to handle exceptions thrown by the target method. This advice is used whenever you want to handle certain exceptions in a different way. Listing 18. Throwing the exception will force the spring container to bypass the execution of the method. No need to panic"). We need to implement one of the following two methods:
void afterThrowing(Throwable excepton) or void afterThrowing(Method method. we are either proceeding to the actual method invocation or throwing an exception. If this the case. The interface we use to create this advice is the ThrowsAdvice. Following is a simple implementation of this advice.out. i.Chapter 18
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return null. import java. This is where the throws advice offers support. Object[] params.aop. This is marker interface that will not have any methods to implement. Object[] params. The parameter to the invoke() method. Using this parameter we can get all the information about the target method. public class AccountNotFoundAdvice implements ThrowsAdvice { public void afterThrowing(Method method. Object target. the invocation parameter is the important one here.e.springframework. Object target.Method.reflect. we used the getArguments() method to read the first and second arguments which happen to be the username and password.4d (AccountNotFoundAdvice.example3. Based on the validation.
Throws Advice
One thing common with the previous three advices is that if any of the advice throws an exception. } }
Let’s now see how the above code works.
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.Throwable exception)
The second method is the widely used one as it also allows us to get the method information. import org.println("Exception Handled. the target method execution is bypassed.

There is absolutely no use in writing the above advices if we cannot inject them in the code. The most popular static point cut is the NameMatchMethodPointCut. this pointcut simply looks for a matched target method name based on the naming pattern specified in the bean definition. we set the property as. In such cases we can list all the method names as shown below
<property name=”methodNames”> <list> <value>testProfile</value> <value>getAccount</value> <value>calculateBalance</value> </list> </property>
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.
<property name=”methodName”> <value>test*</value> </property>
If we only need to apply the advice to just one method. if we need to apply an advice to all the methods whose name starts with “test”. we need to see how we can inject these advices into the real code. For instance. This is where pointcuts come to our rescue. Once the advices are created.
Static Pointcuts
Spring supports both static and dynamic pointcuts. This chapter only covers static point cuts. 1. methodName 2. it’s the pointcut that specifies where the advices needed to be applied.Chapter 18
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} }
Now that we know different types of advices we need to create based on the context. This class has the following two important properties that we normally use to specify the method name patterns. methodNames The first property is used to specify either a single method or a pattern of methods. we need to specify the complete method name as shown below:
<property name=”methodName”> <value>getCustomerProfile</value> </property>
The second property is used when the method names don’t follow a particular pattern. As the name suggests.

>chapter18>javac spring.java
3.springframework.example3.example3.5 from 1234 to5678 Entered into createAccount in springbeans.aop. NameMatchMethodPointcutAdvisor"> <property name="mappedName"> <value>*Account</value> </property> <property name="advice"> <ref bean="logging"/> </property> </bean>
This pointcut specifies that the logging advice should be applied to methods ending with the word “Account”. This evaluates to createAccount() and changeAccount() methods in the implementation class.AccountServiceImpl Created Account Entered into changeAccount in springbeans. The last one is the accountService bean that defines the target object and the list of all the pointcuts. Compile the classes as
C:/>….AccountServiceImpl Updated Accounts Successfully Exiting method changeAccount with a return value null
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.example3.xml
2.support.AccountServiceTest
The output the above code will be
Transfered 200. 3. Steps to run the program 1.>chapter18>javac spring\example3\*. Save all the Java files in the following directory
c:\JavaTraining\chapter18\spring\example3 c:\JavaTraining\chapter18\spring\mybeans. Execute the test class as
C:/>…. This is the bean that will be retrieved by the test class to invoke the business functions.Chapter 18
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<bean id="beforeInterceptor" class="org.

Most of the JDBC code will often be cluttered with loading the drivers. JDBC is the underlying technology for this module. the DAO module has crept into almost all the J2EE applications that use persistence. initializing transactions. For these statements to work.example3. The next module we will look at is the DAO module which is undoubtedly the reason for the success of spring framework. and you’ll see the following result due to AuthenticationAdvice. Just like with Struts framework in which Servlet and JSP are the underlying technologies. we cannot
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. every program needs to execute certain statements before arriving at a point where it can execute the SQL queries.Exception: Unable to change
This completes all the important things you need to know in the core module. These are the fixed parts of the application.
Invalid Entered Created Entered Updated Exiting Credentials into createAccountin springbeans.Chapter 18
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Try changing the username to any other value. managing the connections and statements etc.
DAO Module
We now arrived at a point where we begin to start learning the most widely used spring module. For the last one year or so. and allowing the application developer to concentrate on the important things such as executing the SQL query statements. You might wonder why we wasted all the time learning JDBC which is also used for building persistence applications.example3. The DAO module fundamentally simplifies the persistence application development by abstracting all the low level and fixed parts of the program such as loading the drivers. If you look back at the JDBC applications. creating connections. At this point you should be comfortable working with spring’s aspects and advices along with the factory classes used for wiring the beans.AccountServiceImpl Exception Handled.lang.AccountServiceImpl Created Account Entered into changeAccountin springbeans.example3.AccountServiceImpl Account into changeAccountin springbeans. Once again we need to remember that spring is a framework which like any other framework doesn’t reinvent the wheel but tries to build a better wheel.AccountServiceImpl Accounts Successfully method changeAccountwith a return value null
Try changing the string “Checking” to “Checkings” and you’ll see the following result due to AccountNotFounfAdvice
Invalid Credentials Entered into createAccountin springbeans. creating statements etc. The only variants are query processing statements.example3. No need to panic Exception in thread "main" java.

it also defines several utility methods for executing the SQL queries in a flexible manner.. database schema and all that good stuff. Spring’s DAO module uses something called dataSources and templates for working with databases. Spring comes with a class called JDBCTemplate which is the core DAO class that encapsulates all the crappy code like loading the drivers. Thankfully with DAO module we no longer have to worry about these anymore. let’s see how we configure a data source in the application context XML file.apache. Once the data source bean is defined. we are now ready to work with the Spring’s JDBC templates for database operations. it will also manage the database transactions if needed by the application. Data sources are special beans that are typically defined in application context XML file and are used by spring’s JDBC templates to work with databases.
<bean id="dataSource" class="org. spring uses BasicDataSource to define a data source. Let’s see what these are.BasicDataSource"> <property name="driverClassName"> <value>${driver}</value> </property> <property name="url"> <value>${url}</value> </property> <property name="username"> <value>${userID}</value> </property> <property name="password"> <value>${password}</value> </property> </bean>
As you can see from the above bean definition. we initialized the properties from the property file. This is the pet name given to those objects that encapsulate persistence logic. username. Not only this.commons.Chapter 18
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ignore the fixed parts of the code. Let’s understand the important methods this class has before we start writing some examples using it. In the above definition. The word DAO stands for Data Access Object.dbcp. Not only this. Before we look at the templates.
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. This class requires certain properties to be initialized such as driver class. It will take care of the fixed parts of persistence logic behind the scenes and allows the programmer to only concentrate on executing the query statements. creating connections etc. By the end of this section you’ll agree with me as to why many companies are moving towards using Spring DAO module for data persistence. Following bean definition shows the data source configuration. password.

Listing 18. PreparedStatementSetter pss)
These methods are used to execute the non-SELECT sql queries like INSERT. DELETE etc. following is what it says about JDBCTemplate class. UPDATE. Let’s start writing examples using the data sources and templates with the above methods.
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. initiating iteration over ResultSets and catching JDBC exceptions and translating them to the generic. This method as you might have guessed is normally used with SELECT queries. more informative exception hierarchy defined in the org.example4. Object[] params) int upate(String sql. public class Customer { String name. Object[] args. leaving application code to provide SQL and extract results. Take a look at the code in listing 18.dao package. It simplifies the use of JDBC and helps to avoid common errors. Integer age. The RowCallbackHandler interface extracts values from each row of a ResultSet. Code using this class need only implement callback interfaces. Following are the important methods defined by the JDBCTemplate class.5. String country.springframework.
List query(String sql. giving them a clearly defined contract. String zip. The second form of update() method is another convenient form which uses the PreparedStatementSetter class to populate the prepared statement.Chapter 18
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According to spring’s documentation.5a (Customer.
int update(String sql. providing SQL and any necessary parameters. However there are tons of other methods in the JDBCTemplate class and you are free to explore them. The RowMapper interface is a callback interface which will be called for every row returned by the query. The above three methods are the most commonly used methods. It executes core JDBC workflow. String state.java) Simple Java bean
package spring. Integer ssn. The PreparedStatementCreator callback interface creates a prepared statement given a Connection provided by this class. RowMapper mapper)
This method takes the query and query arguments and returns a list of objects (records). This class executes SQL queries or updates.

The next bean defines a datasource object by supplying the database information as shown below
<bean id="dataSource" class="org. Following shows the template definition.core.springframework.BasicDataSource"> <property name="driverClassName"> <value>${driver}</value> </property> <property name="url"> <value>${url}</value> </property> </bean>
The third bean defines the template object.dbcp.springframework.apache.apache.
<bean id="jdbcTemplate" class="org. The first bean loads the property file jdbc.JdbcTemplate"> <property name="dataSource"> <ref bean="dataSource"/> </property> </bean>
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.dbcp.jdbc. As I said before.Chapter 18
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</bean> <bean id="dataSource" class="org.properties that contains the database information.example4.JdbcTemplate"> <property name="dataSource"> <ref bean="dataSource" /> </property> </bean> <bean id="customerDAO" class="spring.core.CustomerDAO"> <property name="jdbcTemplate"> <ref bean="jdbcTemplate" /> </property> </bean> </beans>
If you observe the above code.commons.BasicDataSource"> <property name="driverClassName"> <value>${driver}</value> </property> <property name="url"> <value>${url}</value> </property> </bean> <bean id="jdbcTemplate" class="org.commons. this object will simply reference the datasource bean. we defined a Customer bean with properties whose values will be inserted into the database table. The best way to understand the above code is by looking at the beans definitions.jdbc. The important thing in this definition is the class attribute which should be the spring’s built in class BasicDataSource.

jdbcTemplate. No wonder why people are getting addicted to this module.args).?. the template property will be set automatically and we can start invoking the persistence methods on it.customer.getAge().close().setString(…) int status = stmt. stmt. String sql = “INSERT INTO CUSTOMERS VALUES(?. Object args[] = new Object[]{customer. This is exactly what is done by the following DAO bean definition:
<bean id="customerDAO" class="example4.update(sql.Driver”). The above code will insert the data into the CUSTOMERS table.getName().
See how clean the above code is.?)". If you look at the DAO class.customer. customer. the DAO bean must set this property before performing the SQL operations.prepareStatement(sql). Now look at the test class.customer. This method invokes the update() method in the JDBCTemplate class and passes the SQL query along with the array of parameters as shown below:
String sql = "INSERT INTO CUSTOMERS VALUES(?.CustomerDAO"> <property name="jdbcTemplate"> <ref bean="jdbcTemplate"/> </property> </bean>
With the above definition.?. con.?. when the DAO bean is retrieved using application context.getState(). The DAO class defines one method insertCustomer() that takes the customer bean as the parameter.Chapter 18
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Once the template bean is defined.?)".?.executeUpdate().?. Without spring.
Therefore.forName(“com.?. stmt.?.getConnection(“blab la “). Connection con = DriverManager.
See the number of lines we reduced and more than that how we made the code look cleaner. PreparedStatement stmt = con.getCountry()}.mysql.jdbc. It simply gets the DAO bean and passes the Customer bean with all the data which will be inserted into the table. no loading drivers and all that crappy stuff.?. the equivalent JDBC code will look as shown below:
Class. we are now ready to use it for database operations.getZip(). it defines the following property:
private JdbcTemplate jdbcTemplate.
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.setString(…) stmt. No creating connections.close(). customer.getSsn().

Chapter 18
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Steps to run the program 1.VARCHAR.?)".update(sql.Types. Compile the classes as
C:/>….getSsn().getZip().VARCHAR}.customer. people will use the following update method to also specify the column types.?.INTEGER.example4. Sometimes to be safer. ssn NUMERIC(10).CustomerDAOTest
Make sure you put the MySQL driver in the classpath before running the program.customer. Age NUMERIC(2).?.Types.INTEGER.java
3.>chapter18>javac spring. Types. customer.?.
String sql = "INSERT INTO CUSTOMERS VALUES(?.getAge(). customer. country varchar(20) )
You’ll see the record inserted into the table. Also create the following table in your MyDB database schema:
CREATE TABLE CUSTOMERS( Name varchar(50).getCountry()}. Execute the test class as
C:/>….Types. zip varchar(10).?. Object args[] = new Object[]{customer.getName().customer. state varchar(10).properties
2.Types.>chapter18>javac spring\example4\*.types).args.
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.VARCHAR. Save all the Java files in the following directory
c:\JavaTraining\chapter18\spring\example4
Save the bean definitions file as
c:\JavaTraining\chapter18\mybeans.getState().xml
Save the property file as
c:\JavaTraining\chapter18\jdbc. jdbcTemplate.VARCHAR. int types[] = new int[]{Types.

getInt(2))).query(sql.setName(rs. Execute the test class and you’ll see all the records displayed. }
Now look at the DAO class. This method should read the record. customer. What will happen behind the scenes is that spring container will invoke the handler method for every record. gather all the customer objects and returns them as a List.
}
Look how simple the method is. customer. return customer. they are less frequently used. return customers. The RowMapper interface defines the following callback method that the spring container will invoke for every record returned by the query.getString(1)). int arg1) throws SQLException
The first thing we need to do is write a class and implement the above method.getString(5)). customer.setAge(new Integer(rs.new MyHandler()). customer. We can then iterate over the list and display the results. customer. We added a method named getCustomers() as shown below:
public List getCustomers(){ String sql = "SELECT * FROM CUSTOMERS". If you are interested in leaning other modules like Web.setZip(rs. List customers = jdbcTemplate. customer. 99% of the time we write code similar to our examples. It passes the SQL query along with the object of the handler class that implemented the RowMapper interface.Chapter 18
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implements this interface. Though spring has several modules. ORM modules.setCountry(rs.getInt(3))). This is exactly what we did in the test class.getString(4)). This completes all the important things you need to know in spring framework.setSsn(new Integer(rs. In real world applications. populate the necessary JavaBean which in our case is the customer bean and return it as shown below:
public Object mapRow(ResultSet rs.getString(6)). int arg1) throws SQLException { Customer customer = new Customer().
public Object mapRow(ResultSet rs.setState(rs. I’d recommend you to read a good
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.null. Try playing with the above two examples to get a good feel for this module. This is all you need to know about DAO module.

The DAO module uses the notion of data sources and templates for persisting data to databases. DAO module etc. Which of the following class is used for loading the beans? a) BeanContext b) BeanFactory 2. you are in pretty good shape. Which of the following class is used to load the bean properties from property files? a) BeanFactory b) ApplicationContext 3.
Time to play 50-50
1. The DAO module hides all the fixed parts of the program like loading drivers. if you understand just the DAO module. The important factory classes are the BeanFactory and ApplicationContext. Which of the following element in the beans definition XML file defines a bean? a) bean b) pojo
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.
Summary
Spring is an open source framework for building enterprise applications.. Spring‘s DAO module is the widely used module for building persistent applications. creating connections etc and allows programmers to only work in the variant parts of the program like executing the SQL queries. Spring’s core module includes factory classes for wiring the beans. The AOP features of spring allow applications to declaratively specify the services that business modules need to leverage. Spring framework includes several modules like core module.Chapter 18
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spring book. web module. Spring uses plain vanilla like objects for encapsulating business logic as opposed to heavy weight components like EJB’s. Trust me. Spring uses inversion of control to promote loose coupling of components.

Question: List the different types of advices spring supports? Answer: Before advice. Which of the following spring class is used to define data source? a) DataSource b) BasicDataSource 7.Chapter 18
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Interview Questions
4. Question: Which class in DAO module is used for executing SQL queries? Answer: JDBCTemplate Question: What is inversion of control? Answer: Refer to page 553. Which of the following spring class is used for executing the SQL queries? a) Template b) JDBCTemplate
Interview Questions
Question: What are the important factory classes used for loading beans? Answer: BeanFactory and ApplicationContext Question: What is the difference between BeanFactory and ApplicationContext classes? Answer: ApplicationContext class can be used to load the bean properties from property files and also support internationalization while BeanFactory cannot. Around advice and Throws advice. Which of the following allows to specify which advices should be applied to business methods? a) pointcut b) joinpoint 6. After advice. Which of the following advice is used for intercepting the method call and allows taking decision whether or not to execute the method? a) Before advice b) Around advice 5.
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xerces-2.6rc1.jar.jar.cglib-2.6.7.1. By the end of this chapter you’ll understand how to use hibernate framework for mapping objects to relational database systems and how to retrieve objects from database using hibernate query features. Copy the following Jar files in the Hibernate installation directory to the “j2eelib” directory.2.0.asm-attrs. http://www.jar. Download the Hibernate framework at the following URL.jar. ehcache-1.jta.
Chapter Goals
Understand the importance of ORM Understanding hibernate components.jar.asm. Understand Polymorphic Associations Understand Hibernate Query Language (HQL)
Environment Setup
1.jar.mysql-connector-java5.
hibernate3. Hibernate.jar.11.jar.6. Add all the above Jar files to the classpath in the batch file.3.jar. Persisting JavaBeans Understanding Associations.jar
3. Make sure to include MYSQL Jar in the classpath.org/6.1.dom4j-1.antlr-2.jar.1.3-bin.log4j-1.hibernate.html 2.2.
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.Chapter 19
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Chapter Goals
Chapter 19
Hibernate
This chapter introduces you to one of the most popular open source ORM solution.

There are infinite numbers of free and commercial persistence solutions available in the market. Instead of drilling into the details of why they are not accepted. let’s work with the problem we have in our hand. Do you see a mismatch here? I am sure you got the point.e. and we don’t need any technologies. In some applications where complex SQL processing is involved Spring JDBC is the way to go. It is important to understand that there has never been an end and will never be an end to the emergence of persistence technologies. whatever may be the reason. This is the problem. Choosing a solution not only depends on how well it works but also depends on other factors like budget. The main challenge with persistence is the “mismatch” that exists between databases and Java application objects. The problem with any relational database is that it stores the data in the form of tables which are two dimensional in nature. There is no point in using a highly sophisticated technology just to persist data to couple of tables. Java applications represent the data in the form of objects whose structure is hierarchical in nature (i. adaptability and several others. Some applications may go for freeware solutions and some others for commercial ones. Every new persistence technology has proven to be somewhat better and flexible than the older ones. The problem is mapping Java objects to tables in
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. In the last few chapters we have seen couple of persistence technologies like JDBC and Spring.. Over the years several persistence technologies have emerged to provide a better and flexible way of persisting the data to databases. then the problem is solved. All this is to say that it is the application demand that should determine which technology to use. It all boils down how much persistence our application needs and how much complexity is involved during persistence. Every technology has its own strengths and weaknesses. However. There is no way you can directly map the Java objects to tables in the database. right? Though there are such object databases available in the market. they are not accepted by the industry in the same way relational databases are accepted. This should give you some idea about how important data persistence is to the success of any application. like a tree which is multi-dimensional).Chapter 19
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Introduction
Introduction
Data persistence is one of the most critical part of any enterprise application. However if we have an “Object Relational databases” that can store the objects directly. we need to find the persistence solutions that can be easily integrated into the Java applications. Applications that don’t persist data are practically less useful and cannot service the customers well enough to retain them. We simply cannot rule them out completely. Every now and then some new persistence solution will pop up and we will be forced to use it. right? Since we are in the world of Java and J2EE.

we should spend more time developing the business logic than working with SQL queries to map the data to database tables. We as Java developers are more comfortable working with Java objects than working with SQL operations to the databases. an EJB container is a must to run the entity beans which makes the application heavier and also consumes lot of resources thereby affecting the application performance.
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. This eventually boils down to how well the persistence layer is designed. Hibernate has proven its capability by providing solutions to complex persistence problems. it involves some level of complexity. This is why we are interested in learning this framework. Any persistence solution that maps objects to tables in database is called as ORM (Object Relational Mapping) solution. As a business logic developer. Though entity beans served the J2EE community for a long time. right? If the design is good. This will save whole lot of time and the application developer can sleep peacefully only worrying about the Java application objects and not the underlying tables. This is an open source persistence framework which is built using XML and Java. Though it not impossible to map the data in Java objects to tables. using JDBC or any other technology doesn’t make any difference. most real world applications started to get away from them due to the complex O/R mappings. One such solution that has become very popular these days is the Hibernate framework.Chapter 19
Hibernate
Introduction
relational database. Historically designers suggested the idea of using EJB entity beans using CMP as a persistence solution. Within short period of time. Our goal as a developer is to persist the data to the database at any cost. Hibernate is one such ORM solution that sits in between Java applications and database as shown in the following figure. The primary goal of any persistence solution is to handle all the persistence mappings by itself behind the scenes by acting as a middleware between Java application and the database and thereby eliminating the burden of complex manual mapping from the application developer. companies started looking for a light weight persistence solution that not only makes life easy but also eliminates the EJB container. This has now become a designers concern and not the developers concern. The question is how much effort we need to put for the same. Moreover. So. The application developer should be responsible to understand the underlying table structure and then write the complex SQL queries to persist the data to the right tables. Historically this is what we were doing.

I can simply set the address object to null.2 Mapping mismatch
As you can see from the above figure. For every operation that I do with the objects. This is a 5:2 mismatch. let’s take a simple example and analyze some persistence details. Both the above tables should be related with each other through a foreign key to associate the account information with the customer information.
Why do we need ORM solution?
To better understand the importance of ORM solution. without wasting any further time.Chapter 19
Hibernate
Why do we need ORM solution?
Java Application
Hibernate ORM Solution
Database
Fig 19. Take a look at the following figure.1 Java and Hibernate
As you can see from the above figure. to delete the address.
Customer
Address
Account
CUSTOMERS
ACCOUNTS
Credit Card
DDA
Fig 19. we have five objects to represent the customer’s data in two tables namely CUSTOMERS and ACCOUNTS. let’s see how Hibernate simplifies our life. Let’s say we have the Customer and Address object represent the data in CUSTOMERS table. For instance. but at the same time I need to execute the following query:
UPDATE CUSTOMERS SET ADDRESS=null WHERE SSN=213213
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. and the remaining objects represent the data in ACCOUNTS table. I need to manually execute the equivalent SQL query. Java applications use Hibernate to map Java objects to Tables in relational database. So.

Let me tell you one thing here. This may lead to some confusion to developers. This is where we need a reliable ORM solution like Hibernate which can take care of creating and executing the SQL queries on the fly and behind the scenes. Allows application developers to manage the Java objects and abstracts all the SQL operations from the developer. 5. Offer loose coupling of Javabeans with the underlying tables though the use of XML as the medium of communication. followed by the record in the CUSTOMERS table due to the foreign key relationship. Hibernate ORM solution provides us with a clean API for persisting data objects. object structure and database structure will be completely apart and persisting and loading data to and from the tables get whacky and error prone. What I will do in
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. One is the user defined XML file that contains all the mapping information.xml file that contains all the database configuration information along with the locations of user defined mapping XML in step 1. 4. 2. 1. if I want to delete the customer. Understanding Hibernate is somewhat tough than the previous ones since all the concepts are tied with the ORM concepts. deleting an object in this case is not a SQL DELETE because address is part of the customer record and we can only update it. A standard hibernate.cfg. Consumes less resources there by improves the overall performance of the application. Provides a dialect (Hibernate Query Language aka HQL) to specify the SQL operations. 3. 2.
Typical Hibernate Components
Hibernate commonly uses two XML files for persisting data in Java objects. Provides an XML based mapping tool to map the Java bean properties to the columns in relational tables. 2. then I need to delete the record in ACCOUNTS table first.
Features of Hibernate ORM Solution
1. Offers significant amount of flexibility in designing persistence applications. 3. Like wise.
Advantages of Hibernate ORM Solution
1. It’s a light weight ORM solution and doesn’t even need a application server or container to run the applications.Chapter 19
Hibernate
Why do we need ORM solution?
So. In real world applications.

we map the bean property ssn.Chapter 19
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Mapping Beans to Tables
1. The only tricky part is the id element. The name of the XML configuration file should be hibernate.hbm. The details about database like Driver. Listing 19. 1. since we want to use SSN as the primary key in the table. In our case. If the property names and column names match each other (case insensitive) then we don’t even have to specify the column attribute as shown below. Specifies the bean properties to the column names.xml.cfg. 3. Every table that we map using the class element must have a primary key column and id element must be used to map a bean property to the primary key. Though there are several ways to specify this information.
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. we will use the XML style. Every example we write will have a new mapping XML file. every property element also specifies the column name.xml is the mapping file just for this example.1c shows this file.
<property name=”firstName” />
The example1. class=”native” means Hibernate should generate the primary key.Customer" table="CUSTOMERS1">
2. See below: class=”assigned” means Hibernate should use the bean property value as the primary key class=”increment” means Hibernate should use a unique number as the primary key and increment it for every record. The id element should also specify a generator child element with a class attribute that tells the Hibernate as to who will pass the value for the primary key. we need to tell Hibernate about two important things. The location of the example mapping files. Before we run the application. URL and all that good stuff 2. Hibernate will automatically figure out that the column name is same as property name.example1. If you notice. Tells Hibernate to map Customer bean to CUSTOMERS1 table using the following line
<class name="hibernate. In our case we used the assigned class since we know that ssn should be used as primary key.

try { SessionFactory sessionFactory = new Configuration().commit().setCountry("UK"). session. customer. customer.setFirstName("James").setSsn(999998). } catch (Exception e) { e. customer.
2. session. we simply created the customer bean and populated all the properties with the values to be persisted to the CUSTOMERS table. Customer customer = new Customer().
Once we completed the above.printStackTrace().setState("LDN"). To insert the customer bean.Chapter 19
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public static void main(String[] args) { Session session = null. Open the Session
session = sessionFactory. customer.save(customer).beginTransaction().setLastName("Bond").openSession(). customer.configure() . session = sessionFactory. customer.getTransaction().setAddressLine1("1111 S St"). customer. } } }
As you can see from the test class. before persisting the object we need to do couple things. 1.buildSessionFactory().buildSessionFactory().configure() .openSession(). Create a SessionFactory object
SessionFactory sessionFactory = new Configuration(). session.beginTransaction().setCity("London"). we simply need to call the save() method on the session as shown below:
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. Begin the transaction
session.
3.

Hibernate will mark the object as “ready to be persisted” but will still not physically write it to the table. Trust me. Similarly.java
3. All you need to do is define the mappings and hibernate will take care of the rest.
When the above method is called. Do you see any SQL in the program? This is the beauty of the Hibernate. we simply have to call the update() method as.
session.
Once the mappings are specified. If all goes well.delete(customer). Save the Java files as
C:/JavaTraining/chapter19/hibernate/example1/Customer.
Try modifying the bean properties and call the update() method. you see the CUSTOMERS table with a record in it. Once we commit the transaction.CustomerTest
Make sure you have completed the environment setup explained at the beginning of the chapter. Steps to run the program 1.xml C:/JavaTraining/chapter19/example1.cfg. and you’ll see the changes in the table. you can delete the customer record using the following:
session. You don’t even have to create the tables.hbm. that’s when hibernate will magically insert the bean properties in the table behind the scenes.
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.update(customer).example1.Chapter 19
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session. you can do what ever with the bean and hibernate will ensure synchronizing the changes in the table. Compile and execute the classes as
C:/JavaTraining>chapter19>javac hibernate\example1\*.java C:/JavaTraining>chapter19>java hibernate. If you want to update the customer record.save(customer). Save the two XML files in the following directory as:
C:/JavaTraining/chapter19/hibernate.java C:/JavaTraining/chapter19/hibernate/example1/CustomerTest.xml
2.

Case 1: XML based configuration This is the scheme we demonstrated in our previous examples where all the database information and mapping files are defined in a XML file named hibernate. For instance.cfg. using Hibernate with EJB requires a different configuration etc. Usually configuring SessionFactory depends on application demands.sourceforge. Also configuration is based on the environment in which Hibernate is used.dtd">
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.0. The haddress and oaddress are the child components of address component.
Configuring Hibernate
One of the striking features of Hibernate is that it can be used in diverse variety of environments. In the next section. With this scheme. So. The SessionFactory class as the name suggests is a factory of session objects. let’s see the different ways we can configure Hibernate. Hibernate uses Configuration class to configure the session factory. Before we open the session.xml) XML configuration file
<?xml version='1.cfg.3a (hibernate. you can map any complex bean structure to the underlying table columns.net/hibernate-configuration-3.xml.0' encoding='utf-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://hibernate. Following listing shows the configuration XML file: Listing 19. it is every important that we understand some of the important ways of configuring Hibernate.Chapter 19
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</component> </class>
As you can see from the above definition. we need to configure the SessionFactory with the underlying database(s). By using nested components. Some applications may use just one database and others might use several databases for persistence. we nested component elements. Hibernate uses a class called SessionFactory for persisting and loading data objects to and from the database. let’s see some important configuration schemes for the SessionFactory. the XML should be placed in the classpath of the application which is typically the root directory of the application. Therefore.

driver_class"> com..Chapter 19
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<hibernate-configuration> <session-factory> <property name="hibernate.
You can use this type of configuration if you prefer to use XML style.openSession().save(customer).hbm.MySQLDialect </property> <property name="hibernate.cfg. we need multiple configuration XML files such as security.connection.connection.dialect.properties file.
The above scheme works for single session factory.hbm2ddl.buildSessionFactory().
Once the factory instance is obtained.cfg.xml”). we can open the session objects and start persisting objects as shown below:
session = factory.
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. session.connection.hibernate.xml etc. What if we want to work with multiple databases? In such cases. our application can then create the SessionFactory object as shown below:
SessionFactory factory = new Configuration().Mapping files --> <mapping resource="example3. we can obtain the session factory objects as shown below:
SessionFactory factory1 = new Configuration().mysql.auto">update</property> <!-. Case 2: Using hibernate.buildSessionFactory().connection. with each configuration file containing information about a particular database.connection.cfg.configure(“financial.cfg.jdbc.password"></property> <property name="hibernate.pool_size">10</property> <property name="show_sql">true</property> <property name="dialect"> org.buildSessionFactory(). SessionFactory factory2 = new Configuration().xml" /> </session-factory> </hibernate-configuration>
With the above configuration. financial. In such cases.Driver </property> <property name="hibernate.xml.xml”).configure().configure(“security.url"> jdbc:mysql://localhost:3306/MDIT </property> <property name="hibernate.username">root</property> <property name="hibernate.

transaction.load(new FileInputStream(“security. we can obtain the session factory object as shown below:
SessionFactory factory = cfg1.properties file in the classpath.transaction.jdbc.MySQLDialect
However. if you want to work with multiple databases. except that it uses hibernate. cfg1.transaction. you can obtain the SessionFactory object as shown below:
SessionFactory factory = new Configuration().
Once the mapping resources are added. cache properties.factory_class= org.Driver hibernate. The property file will then list the properties as shown below: hibernate.Chapter 19
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This configuration scheme is much the same as the previous one.hibernate. Once you have this property file in the classpath. right? Following is how we do:
cfg1. We also need to specify the location of the mapping files.hibernate.hbm.connection.url = jdbc:mysql://localhost:3306/MyDB hibernate.addResource(“example1. if you are using JNDI based datasources. cfg1.hbm.connection.properties”)).buildSessionFactory().connection.addResource(“example2.driver_class = com.JbossTransactionManagerLookup hibernate.mysql.manager_lookup_class= org. connection properties etc.transaction.setProperties(p).
Again.properties
# JDBC Properties hibernate.hibernate.xml”). then you need to use the following properties:
hibernate.connection.xml”).JTATransactionFactory hibernate. Configuration cfg1 = new Configuration().
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.connection.hibernate.password = root hibernate.dialect = org. you need to create multiple property files and load them as shown below:
Properties p = new Properties().MySQLDialect
There are also several other properties that you can define like transaction properties.username = root hibernate.dialect.datasource = jdbc/MySQL hibernate.dialect.
There is one more thing we missed here.dialect = org. p.buildSessionFactory().

Fig 19.buildSessionFactory(). (i.dialect.In the next section we will see the most important aspect of ORM. both A and B can see each others changes. cfg1.username".setProperty("hibernate.dialect". cfg1."org.e B cannot see the changes in A). SessionFactory factory = cfg1.
The advantage with programmatic configuration is that you can pass the property values at runtime. Consider that we have two parties namely A and B that are associated with each other. You’ll simply specify the configuration in the program itself as shown below:
Configuration cfg1 = new Configuration().hbm."jdbc:mysql://localhost:3306/MyDB") cfg1. cfg1.setProperty("hibernate.connection.Driver").hibernate. cfg1. If the association is unidirectional from A to B.setProperty("hibernate.addResource("example2..hbm. This is a very strong relationship and is normally read as “has a” relationship.mysql.xml")."com. you neither have to use XML file nor have to use the property file. This completes all the important and widely used schemes of configuring hibernate. the associations.
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.3 shows both unidirectional and bidirectional associations. If you prefer XML style then use the Case 1 scheme. cfg1. With a bidirectional association.xml"). This gives your application more flexibility while working with multiple databases.Chapter 19
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You can choose which ever style you want. then A can see changes made to B and not vice-versa.connection.MySQLDialect").driver_class".
Associations
Associations basically indicate that one class retains the relationship to another class over an extended period of time. if you prefer using property files then use the Case 2 scheme. an address is associated with a customer."root"). If you recall our Customer-Address example. Moreover associations can be unidirectional or bidirectional.addResource("example1. Following are some of the examples: Customer has Address Product has a Name A company has employees etc.setProperty("hibernate.url".connection. Case 3: Programmatic configuration In this scheme.

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Unidirectional
A
B
A
B
Bidirectional Fig 19. BOOKS BOOK_ID PUB_ID AUTHOR TITLE 101 123 102 123 PUBLISHERS PUB_ID PUB_NAME 123
The class definition for Publisher class will be as shown below:
<class name="hibernate. 3. 4.example4. it becomes unidirectional.3 Unidirectional and Bi-directional associations
Associations are basically represented in the following four ways: 1. One-to-One Association One-to-Many association Many-to-One association Many-to-Many association
Let’s look at each of these associations one by one. At the same time if only the Books should be able to see the changes made to the publisher. 2.Publisher" table="PUBLISHERS4"> <id name="publisherId" column="PUB_ID"> <generator class="increment" /> </id> <property name="publisherName" column="PUB_NAME" /> </class>
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.
Many-to-One Association
Unidirectional Many-to-One/One-to-many Association The simple example for this association is multiple books belonging to the same publisher.

String publisherName.
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.configure() .new Integer(1)). Publisher pub = book.setTitle("J2EE1"). Therefore. Book book2 = new Book().
The above code will print the publisher name of the Book.println(pub. System.getPublisherName()).buildSessionFactory(). Note: Hibernate will also create the tables for you with all the proper table constraints.example4.setPublisher(pub).commit(). Book book = (Book)session.setAuthor("Phani2"). book2.openSession(). session = sessionFactory. Set books. the publisher should have the knowledge of all the books it published. // Getter and Setters void addBook(Book book){ books. In this example it creates PUBLISHERS4 and BOOKS1 tables that we specified in the xml file as shown below.class.Book" table="BOOKS4">
Test to verify the unidirectional association Test: Given a Book. book2.add(book).beginTransaction().get(Book. session.out.setTitle("J2EE2").Publisher" table="PUBLISHERS4"> <class name="hibernate. book2. session.
<class name="hibernate. These tables will be created only once. Bidirectional Many-to-one/One-to-Many In this case.example4.getTransaction(). and 2 records in the BOOKS4 table.getPublisher().
Execute the test class and you’ll see 1 record inserted in the PUBLISHERS4 table. the publisher class should have a collection property for books as shown below:
public class Publisher { int publisherId.Chapter 19
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book1. display the publisher information
SessionFactory sessionFactory = new Configuration().

display all the Books
SessionFactory sessionFactory = new Configuration(). } }
Look at the addBook() utility method. session.
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.example4. all we need to do is just update the record in the books table to set the publisher using the following update statement.new Integer(1)).commit().openSession().configure() . Publisher pub = (Publisher)session.Book" /> </set> </class>
Note that the set element defines that each book should have the same PUB_ID and there will be a one-to-many association between the publisher and the book.example4. we searched for a publisher whose ID is 1 and then displayed the books. we need to set the inverse attribute to true as shown in the following class definition:
<class name="hibernate. The above code will print all the book objects. Set books = pub.setPublisher(this). But logically speaking.get(Publisher.Publisher" table="PUBLISHERS4"> <id name="publisherId" column="PUB_ID"> <generator class="increment" /> </id> <property name="publisherName" column="PUB_NAME" /> <set name="books" inverse="true"> <key column="PUB_ID" /> <one-to-many class="hibernate. session. If we try to associate a book to a publisher. To make Hibernate just execute the above update statement.getTransaction(). System.Chapter 19
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book.println(books). update BOOKS set PUB_ID=? where BOOK_ID=? However hibernate recognizes two changes.class. one for updating the book and the other for updating the publisher of the book. Test to verify the bidirectional association Test: Given a Publisher.beginTransaction(). session = sessionFactory.buildSessionFactory(). both these operations are same.out.
As you can see from the above code.getBooks().

For instance let’s say we have the following tables: BOOKS BOOK_ID PUB_ID AUTHOR TITLE 101 123 102 123 PUBLISHERS PUB_ID PUB_NAME 123
As you can see from the above table. As can you see from the previous example. Using Primary Key Unidirectional Foreign Key Association This is the simplest of two. Let’s again look at both unidirectional and bidirectional one-to-one associations using the Book and Publisher.Book" table="BOOKS5"> <id name="bookId" column="BOOK_ID"> <generator class="increment" /> </id> <many-to-one name="publisher" column="PUB_ID" unique="true" /> <property name="author" column="AUTHOR" /> <property name="title" column="TITLE" /> </class>
The class definition for Publisher will remain the same as shown below:
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. For example. This association can be implemented in two ways. a one-to-one relationship can be obtained from a many-to-one relationship by eliminating the duplicate foreign key (PUB_ID) in the books table.example5. Using Foreign Key 2. a one-to-one association is derived from a manyto-one relationship by setting the unique property to true for the PUB_ID column in the BOOKS table. with this association.Chapter 19
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Both one-to-many and many-to-one relationships work just the same way. a bidirectional many-to-one also includes a one-to-many association.
One-to-One Association
As the name suggests. In this case. We tell this to hibernate by simply setting the unique attribute to true for the PUB_ID in the class definition for book as shown below:
<class name="hibernate. a book can only be associated with one publisher. The PUB_ID then becomes the foreign key to the PUBLISHERS table primary key. one object (record in one table) should be associated with one and only one object (record in another table). 1.

session.mysql.jdbc. Following is the test code to verify this fact.save(book1).setAuthor("Phani1").executeBatch(ServerPreparedStatement. book2.setTitle("J2EE2").setPublisherName("Wrox").Chapter 19
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<class name="hibernate. if we try to add more than one book to the same publisher it will add the first book and then throws an exception.save(book2). When Hibernate generates the DDL for the tables.setAuthor("Phani2").ServerPreparedStatement.
When you run the above code.jav a:657)
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. Book book1 = new Book().
Publisher pub = new Publisher(). BOOKS5 and PUBLISHERS5. session. book2.setPublisher(pub).save(pub). You’ll then see an exception something like shown below:
Caused by: java.setTitle("J2EE1"). it adds a foreign key constraint to the PUB_ID in the Books table as shown below:
BOOKS2 BOOK_ID: PK PUB_ID : FK AUTHOR PUBLISHERS2 PUB_ID : PK PUB_NAME
With the above relationship.example5. book1. you’ll see one record inserted in the PUBLISHERS2 table. book1.Publisher" table="PUBLISHERS5"> <id name="publisherId" column="PUB_ID"> <generator class="increment" /> </id> <property name="publisherName" column="PUB_NAME" /> </class>
Notice that we are using different table names for this example.setPublisher(pub). and one record in the BOOKS2 table. session. Book book2 = new Book().BatchUpdateException: Duplicate entry '1' for key 2 at com. book1. pub.sql. book2.

example5. session = sessionFactory.openSession().buildSessionFactory(). we added a one-to-one mapping of the book to the publisher using the property-ref attribute.println(book). Book book = pub.commit().configure() . Firstly.out.example5.class. session. it has to be just one book.Publisher" table="PUBLISHERS5"> <id name="publisherId" column="PUB_ID"> <generator class="increment" /> </id> <property name="publisherName" column="PUB_NAME" /> <one-to-one name="book" class="hibernate.get(Publisher.new Integer(1)). String publisherName.getBook(). Publisher pub = (Publisher)session. we need to add the book reference in the Publisher class as shown below:
public class Publisher { int publisherId. session.Book" property-ref="publisher" /> </class>
As you can see from the above definition.beginTransaction(). The only difference is that in this case. the class definition for the Publisher will look as shown below:
<class name="hibernate. Book book.
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. Test to verify the bidirectional association Test: Given a Publisher.getTransaction(). display the associated Book
SessionFactory sessionFactory = new Configuration(). System.Chapter 19
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The test to verify the unidirectional association is same as unidirectional one-tomany/many-to-one association we discussed before. Bidirectional Foreign Key Association Making this association bidirectional is nothing but having the publisher access the book information. This tells Hibernate that the book association in Publisher is the reverse of publisher association in Book. // Getter and Setters for book }
Secondly.

hibernate will insert same value in both the tables. This way we can completely delete the foreign key PUB_ID from the books table. The challenge we have here is that. This is done using the constrained=true attribute as shown in the following publisher class definition:
<class name="hibernate. This means publisher is dependent on the book.example5.
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. Let’s now look at the same using primary key. we first need to generate the primary key of the Book. This is shown by the “book” property in the above definition.
BOOKS3 BOOK_ID: PK PUB_ID AUTHOR
PUBLISHERS3 PUB_ID : PK PUB_NAME
As you can see from the above figure.Publisher" table="PUBLISHERS6"> <id name="publisherId" column="PUB_ID"> <generator class="foreign"> <param name="property">book</param> </generator> </id> <property name="publisherName" column="PUB_NAME" /> <one-to-one name="book" constrained="true" /> </class>
There are three important things we need to understand with the above definition. 1. Unidirectional Primary Key Association With this type of association. both the BOOK_ID and PUB_ID must always be the same. Only then. This is because. and then use its value as the primary key in the publisher table. This completes both unidirectional and bidirectional one-to-one association using foreign keys. both the books and the publishers table will have the same primary key as shown in the following figure. The direction of the association is from Publisher to Book.Chapter 19
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The above test code will display the book information for the publisher whose id is 1. we somehow need to tell hibernate to use PUB_ID in the publisher table as both primary key and also the foreign key of the BOOK_ID in the books table.

commit(). Hmmm. right?.setBook(book1).setTitle("J2EE1"). book1. pub. session.beginTransaction(). I can understand but learn it twice and you’ll get it. Here is a simple trick to remember: Trick: If we have a method invocation as a. then the direction of the association is from a to b.setAuthor("Phani1"). Confusing..
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. we first created the book object and then passed the book to the publisher object. Following is how the test code looks:
session.save(pub). 3. publisher should set the book. where a and b are two objects. The constraint=true attribute tells hibernate that there is a foreign key constraint on the primary key of the publisher table. Should the publisher object set the value for book. A special generator named foreign must be used which tells hibernate to use the value of the primary key on the books table as the primary key value in the publishers table.save(book1).
As you can see from the above code. session. The class definition for the book will be simply be a straight forward mapping as shown below:
<class name="hibernate.Chapter 19
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2. Let me ask you a question. This is very important to understand otherwise you will get an exception.setTest(b). Book book1 = new Book(). pub. or should the book object set the value for publisher? Since the association is from publisher to book. Publisher pub = new Publisher().Book" table="BOOKS6"> <id name="bookId" column="BOOK_ID"> <generator class="increment" /> </id> <property name="author" column="AUTHOR" /> <property name="title" column="TITLE" /> </class>
The test code for adding records to the table should be done carefully.setPublisherName("Wrox"). book1.example5. session.getTransaction().

Many-to-Many Association
The classical example for this association is the one between Topics and Subscriptions.Book" table="BOOKS6"> <id name="bookId" column="BOOK_ID"> <generator class="increment" /> </id> <property name="author" column="AUTHOR" /> <property name="title" column="TITLE" /> <one-to-one name=”publisher” class=”hibernate. Publisher pub = book. This is one of the rarely seen associations. (or) Book book = (Book)session.new Integer(1)).Chapter 19
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Bidirectional Primary Key Association Making the previous association as bidirectional is very simple.example5.new Integer(1)).getPublisher().class.
This completes every thing we need to know about one-to-one association. You can now use either of the following test code to get the details of book and publisher:
Publisher pub = (Publisher)session.example5.Publisher”/> </class>
The above tells hibernate to also retrieve the publisher while retrieving the book.get(Book. a single topic will have more than one subscriber and a single subscriber can subscribe to more than one topic. The last one is the many-to-many association. As you know. This time we need to add a publisher property in the book object using a one-to-one association as shown below:
<class name="hibernate. Let’s see how it looks. Following figure shows this association:
Topic 1 Subscriber 1
Topic 1
Subscriber 2
Topic 1
Subscriber 3
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.get(Publisher. Do you see a many-to-many relationship here? I am sure you did. Book b = pub.class.getBook().

Set subscribers. In this case.Chapter 19
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Implementing a many-to-many relationship requires an additional table called the link table. // Getters and Setters }
With unidirectional association. Unidirectional Many-to-Many association In this case. FK SUB_ID: PK. String name. String topicName. Set topics. The link table basically contains the primary keys of both the tables as shown in the following diagram:
TOPICS TOPIC_ID : PK TOPIC_NAME TOPIC_SUB TOPIC_ID : PK. There is no way you can implement this scenario using two tables. FK
SUBSCRIBERS SUB_ID : PK NAME
Let’s see both unidirectional and bidirectional many-to-many associations. // Getters and Setters }
The subscriber class will define a collection of topics objects as shown below:
public class Subscriber{ int sid. the class definition for Topic class will look as shown below:
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. a topic will have the knowledge of all the subscribers and not vice-versa. the Topic object will define a collection of subscriber objects as shown below:
public class Topic{ int topicid.

With the above data. topic. sub3. it inserts the primary keys of the both TOPICS and SUBSCRIBERS table. when we do a search for the topic using the topic id.openSession(). following is how the contents of three tables will look like: TOPICS TOPIC_ID TOPIC_NAME 1 Movies TOPIC_SUB TOPIC_ID SUB_ID 1 1 1 2 1 3 SUBSCRIBERS SUB_ID NAME 1 James 2 John 3 Sara
As you can see from the above link table. session. session. } catch (Exception e) { e. topic.configure() . } } }
As you can see from the above code.Chapter 19
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Session session = null. sub1.setTopicName("Topic 1").save(sub2).class.buildSessionFactory().setName("Phani2").commit(). Subscriber sub1 = new Subscriber().beginTransaction(). try { SessionFactory sessionFactory = new Configuration().getTransaction(). hibernate will return all the three subscribers with the following code:
Topic topic = (Topic)session. topic. session. topic. Subscriber sub3 = new Subscriber(). session.addSubscriber(sub1). we created 3 subscribers and added them to one topic.printStackTrace(). When we execute the above code. Topic topic = new Topic().addSubscriber(sub3).get(Topic.save(sub1).setName("Phani1").
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.save(sub3).save(topic). session = sessionFactory.addSubscriber(sub2).setName("Phani3"). session. sub2.new Integer(1)). session. Subscriber sub2 = new Subscriber().

new Integer(1)). However. Let’s now look at the bidirectional many-to-many association. Bidirectional Many-to-Many association In this case.example6. t1. t2.
With the above code.save(t2) .save(t1) .class. Topic t2 = new Topic(). // Get the Subscriber Subscriber sub = (Subscriber) session. sub.addTopic(t1).save(sub) . we need to update the class definition of the subscriber to include the topics property as a set element as shown below:
<class name="hibernate. we need to add the inverse=”true” attribute to tell hibernate to synchronize both the ends of the association.getSubscribers()). sub. let’s have James subscribe to two new topics as shown below:
Topic t1 = new Topic().addTopic(t2).println(topic. session.setTopicName(“Java”). The class mapping for the Topic will remain the same as the one with unidirectional association.Chapter 19
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System.setTopicName(“J2EE”).out.Subscriber" table="SUBSCRIBERS6"> <id name="sid" column="SUB_ID"> <generator class="increment" /> </id> <property name="name" column="NAME" /> <set name="topics" table="TOPIC_SUB" inverse=”true” > <key column="SUB_ID" /> <many-to-many column="TOPIC_ID" class="hibernate. session.get(Subscriber.Subscriber" /> </set> </class>
As you can see from the above definition. session.example6. With these class definitions. this is simply the reverse of the subscribers property in the Topic class